Lecturer:D.Myagmarsuren,
NUM
General
chemistry
Chemistry is such a broad subject
and one so full of detail that it is easy for a newcomer to find it somewhat
overwhelming, if not intimidating. The best way around this is to look at
Chemistry from a variety of viewpoints:
- How Chemistry relates to other sciences and to the world in general
- What are some of the fundamental concepts that extend throughout Chemistry?
- What are some of the major currents of modern-day Chemistry?
The scope of chemical science
Chemistry is too universal and
dynamically-changing a subject to be confined to a fixed definition; it might
be better to think of chemistry more as a point of view that places its
major focus on the structure and properties of substances— particular
kinds of matter— and especially on the changes that they undergo.
In some ways, physics might be
considered more "fundamental" to the extent that it deals with matter
and energy in a more general way, without the emphasis on particular
substances. But the distincion can get pretty fuzzy; it is ultimately rather
futile to confine any aspect of human endeavour to little boxes.
Chemistry:
the central science
The real importance of Chemistry is
that it serves as the interface to practically all of the other sciences, as
well as to many other areas of human endeavor. For this reason, Chemistry is
often said (at least by chemists!) to be the "central science".
Chemistry can be "central"
in a much more personal way: with a solid background in Chemistry, you will
find it far easier to migrate into other fields as your interests develop.
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Research or teaching not for you? Chemistry is so deeply ingrained into so many areas of
business, government, and environmental management that some background in
the subject can be useful (and able to give you a career edge as a team
member having special skills) in fields as varied as product development,
marketing, management, computer science, technical writing, and even law.
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So
just what is chemistry?
Do you remember the story about the
group of blind men who encountered an elephant? Each one moved his hands over a
different part of the elephant's body— the trunk, an ear, or a leg— and came up
with an entirely different description of the beast.
Chemistry can similarly be
approached in different ways, each yielding a different, valid, (and yet
hopelessly incomplete) view of the subject.
Thus we can view chemistry from multiple standpoints ranging from the theoretical to the eminently practical:
Thus we can view chemistry from multiple standpoints ranging from the theoretical to the eminently practical:
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Mainly theoretical
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Mainly practical
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Why do particular combinations of
atoms hold together, but not others?
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What are the properties of a
certain compound?
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How can I predict the shape of a
molecule?
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How can I prepare a certain
compound?
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Why are some reactions slow, while
others occur rapidly?
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Does a certain reaction proceed to
completion?
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Is a certain reaction possible?
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How can I determine the
composition of an unknown substance?
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Boiling
it down to the basics
At the most fundmental level,
chemistry can be organized along the lines shown here.
- Dynamics refers to the details of that rearrangements of atoms that occur during chemical change, and that strongly affect the rate at which change occurs.
- Energetics refers to the thermodynamics of chemical change, relating to the uptake or release of heat. More importantly, this aspect of chemistry controls the direction in which change occurs, and the mixture of substances that results.
- Composition and structure define the substances that are results of chemical change. Structure refers specifically to the relative arrangements of the atoms in space. The extent to which a given structure can persist is itself determined by energetics and dynamics.
- Synthesis, strictly speaking, refers to formation of new (and usually more complex) substances from simpler ones, but in the present context we use it in the more general sense to denote the operations required to bring about chemical change and to isolate the desired products.
This view of Chemistry is a rather
astringent one that is probably more appreciated by people who already know the
subject than by those who are about to learn it, so we will use a somewhat
expanded scheme to organize the fundamental concepts of chemical science. But
if you need a single-sentence"definition of Chemistry, this one wraps it
up pretty well:
Chemistry is the study of substances;
their properties, structure, and the changes they undergo.
Micro-macro:
the forest or the trees
Chemistry, like all the natural
sciences, begins with the direct observation of nature— in this case, of
matter. But when we look at matter in bulk, we see only the "forest",
not the "trees"— the atoms and molecules of which matter is composed—
whose properties ultimately determine the nature and behavior of the matter we
are looking at.
This dichotomy between what we can
and cannot directly see constitutes two contrasting views which run through all
of chemistry, which we call macroscopic and microscopic.
In the context of Chemistry,
"microscopic" implies detail at the atomic or subatomic levels which
cannot be seen directly (even with a microscope!)
The macroscopic world is the one we can know by direct observations of physical properties such as mass, volume, etc.
The macroscopic world is the one we can know by direct observations of physical properties such as mass, volume, etc.
The following table provides a
conceptual overview of Chemical science according to the
macroscopic/microscopic dichotomy we have been discussing. It is of course only
one of many ways of looking at the subject, but you may find it a helpful means
of organizing the many facts and ideas you will encounter in your study of
Chemistry. We will organize the discussion in this lesson along similar lines.
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realm
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macroscopic view
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microscopic view
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composition
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formulas, mixtures
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structures of solids, molecules,
and atoms
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properties
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intensive properties of bulk
matter
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particle sizes, masses and
interactions
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change (energetics)
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energetics and equilibrium
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statistics of energy distribution
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change (dynamics)
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kinetics (rates of reactions)
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mechanistics
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Mixture
or "pure substance" ?
In science it is absolutely
necessary to know what we are talking about, so before we can even begin to
consider matter from a chemical point of view, we need to know something about
its composition; is the stuff I am looking at a single substance,
or is it a mixture? (We will get into the details of the definitions
elsewhere, but for the moment you probably already have a fair understanding of
the distinction; think of a sample of salt (sodium chloride) as opposed to a solution
of salt in water— a mixture of salt and water.)
The
manufacturer probably claims that their peanut butter
is "pure"; is it really what a chemist would call a "pure substance"?
is "pure"; is it really what a chemist would call a "pure substance"?
Elements
and compounds
It has been known for at least a
thousand years that some substances can be broken down by heating or chemical
treatment into "simpler" ones, but there is always a limit; we
eventually get substances known as elements that cannot be reduced to
any simpler forms by ordinary chemical or physical means. What is our criterion
for "simpler"? The most observable (and therefore macroscopic)
property is the weight.
The idea of a minimal unit of
chemical identity that we call an element developed from experimental observations
of the relative weights of substances involved in chemical reactions. For
example, the compound mercuric oxide can be broken down by heating into two
other substances:
2 HgO → 2 Hg + O2
... but the two products, metallic
mercury and dioxygen, cannot be decomposed into simpler substances, so they
must be elements.
Elements
and atoms
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The definition of an element given
above is an operational one; a certain result (or in this case, a
non-result!) of a procedure that might lead to the decomposition of a
substance into lighter units will tentatively place that substance in one of
the categories, element or compound. Because this operation is carried out on
bulk matter, the concept of the element is also a macroscopic one.
Painting
by Joseph Wright of Derby (1734-97) The Alchymist in Search of the Philosopher's Stone discovers Phosphorus
[link]
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The atom, by contrast, is a microscopic
concept which in modern chemistry relates the unique character of every
chemical element to an actual physical particle.
The idea of the atom as the smallest
particle of matter had its origins in Greek philosophy around 400 BCE but was
controversial from the start (both Plato and Aristotle maintained that matter
was infinitely divisible.) It was not until 1803 that John Dalton proposed a
rational atomic theory to explain the facts of chemical combination as they
were then known, thus being the first to employ macroscopic evidence to
illuminate the microscopic world.
It took almost until 1900 for the
atomic theory to became universally accepted. In the 1920's it became possible
to measure the sizes and masses of atoms, and in the 1970's techniques were developed
that produced images of individual atoms.
Formula
and structure
The formula of a substance expresses
the relative number of atoms of each element it contains. Because the formula
can be determined by experiments on bulk matter, it is a macroscopic concept
even though it is expressed in terms of atoms.
What the ordinary chemical formula
does not tell us is the order in which the component atoms are
connected, whether they are grouped into discrete units (molecules) or
are two- or three dimensional extended structures, as is the case with solids
such as ordinary salt. The microscopic aspect of composition is structure,
which in its greatest detail reveals the relative locations (in two or three
dimensional space) of each atom within the minimum collection needed to define
the structure of the substance.
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Macroscopic
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Microscopic
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Substances are defined at the
macroscopic level by their formulas or compositions, and at the
microscopic level by their structures.
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The elements hydrogen and oxygen
combine to form a compound whose composition is expressed by the formula H2O.
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The molecule of water has the
structure shown here.
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Chemical substances that cannot be
broken down into simpler ones are known as elements. The actual
physical particles of which elements are composed are atoms or molecules.
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Sulfur – the element in its
orthorhombic crystalline form.
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The S8 molecule
is an octagonal ring of sulfur atoms. The crystal shown at the left is
composed of an ordered array of these molecules.
(No, they don't actually move
around like this, although they are in a constant state of vibrational
motion.)
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Compounds
and molecules
As we indicated above, a compound
is a substance containing more than one element. Since the concept of an
element is macroscopic and the distinction between elements and compounds was
recognized long before the existence of physical atoms was accepted, the
concept of a compound must also be a macroscopic one that makes no assumptions
about the nature of the ultimate .
Thus when carbon burns in the
presence of oxygen, the product carbon dioxide can be shown by (macroscopic)
weight measurements to contain both of the original elements:
C + O2 → CO2
10.0 g + 26.7 g = 36.7 g
One of the important characteristics
of a compound is that the proportions by weight of each element in a given
compound are constant. For example, no matter what weight of carbon dioxide we
have, the percentage of carbon it contains is (10.0 / 36.7) = 0.27, or 27%.
Molecules
A molecule is an assembly of
atoms having a fixed composition, structure, and distinctive, measurable
properties.
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"Molecule" refers to a
kind of particle, and is therefore a microscopic concept. Even at the end of
the 19th century, when compounds and their formulas had long been in use,
some prominent chemists doubted that molecules (or atoms) were any more than
a convenient model.
Computer-model
of the nicotine molecule, C10H14N2, by Ronald
Perry
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Molecules suddenly became real in
1905, when Albert Einstein showed that Brownian motion, the irregular
microscopic movements of tiny pollen grains floating in water, could be
directly attributed to collisions with molecule-sized particles.
Finally, we get to see one! In 2009, IBM scientists in Switzerland succeeded in imaging
a real molecule, using a technique known as atomic force microscopy
in which an atoms-thin metallic probe is drawn ever-so-slightly above the
surface of an immobilized pentacene molecule cooled to nearly absolute zero. In
order to improve the image quality, a molecule of carbon monoxide was placed on
the end of the probe.
The image produced by the AFM probe
is shown at the very bottom. What is actually being imaged is the surface of
the electron clouds of the molecule, which consists of six hexagonal rings of
carbon atoms with hydrogens on its periphery. The tiny bumps that correspond to
these hydrogen atom attest to the remarkable resolution of this experiment.
The original article
was publshed in Science magazine; see here for an
understandable account of this historic work.
The atomic composition of a molecule
is given by its formula. Thus the formulas CO, CH4, and O2
represent the molecules carbon monoxide, methane, and dioxygen. However, the
fact that we can write a formula for a compound does not imply the existence of
molecules having that composition. Gases and most liquids consist of molecules,
but many solids exist as extended lattices of atoms or ions (electrically
charged atoms or molecules.) For example, there is no such thing as a
"molecule" of ordinary salt, NaCl (see below.)
Confused
about the distinction between molecules and compounds?
Maybe the following will help:
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A molecule but not a compound - Ozone, O3, is not a compound because
it contains only a single element.
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Ordinary solid salt is a
compound but not a molecule. It is built from interpenetrating lattices
of sodium and chloride ions that extend indefinitely.
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Structure
and properties
Composition and structure lie at the
core of Chemistry, but they encompass only a very small part of it. It is
largely the properties of chemical substances that interest us; it is
through these that we experience and find uses for substances, and much of
chemistry-as-a-science is devoted to understanding the relation between
structure and properties. For some purposes it is convenient to distinguish
between chemical properties and physical properties, but as with most
human-constructed dichotomies, the distinction becomes more fuzzy as one looks
more closely.
This concept map offers a good
overview of the ideas we have developed so far. Take some time to look it over
and make sure you understand all the terms and the relations between them.
For a more in-depth treatment of
much of the material covered here, please see The basics of atoms, moles, formulas equations, and
nomenclature.
Chemical change is defined macroscopically as a process in which new
substances are formed. On a microscopic basis it can be thought of as a
re-arrangement of atoms. A given chemical change is commonly referred to as a
chemical reaction and is described by a chemical equation that has the form
reactants → products
Chemical
change vs. physical change
In elementary courses it is
customary to distinguish between "chemical" and "physical"
change, the latter usually relating to changes in physical state such as
melting and vaporization. As with most human-created dichotomies, this begins
to break down when examined closely. This is largely because of some ambiguity
in what we regard as a distinct "substance".
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Example: dichlorine, Cl2.
Elemental chlorine exists as the
diatomic molecule Cl2 in the gas, liquid, and solid states; the
major difference between them lies in the degree of organization. In the gas
the molecules move about randomly, whereas in the solid they are constrained
to locations in a 3-dimensional lattice. In the liquid, this tight
organization is relaxed, allowing the molecules to slip and slide around each
other.
Since the basic molecular units
remain the same in all three states, the processes of melting, freezing,
condensation and vaporization are usually regarded as physical rather
than chemical changes.
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Example: sodium chloride, NaCl.
Solid salt consists of an
indefinitely extended 3-dimensional array of Na+ and Cl–
ions (electrically- charged atoms.)
When heated above 801°C, the solid
melts to form a liquid consisting of these same ions. This liquid boils at
1430° to form a vapor made up of discrete molecules having the formula Na2Cl2.
Salt dissolves in water to form a
solution containing separate Na+ and Cl– ions to which
are loosely attached varying numbers of H2O molecules. The
resulting hydrated ions are represented as Na+(aq) and Cl–(aq).
Because the ions in the solid, the
hydrated ions in the solution, and the molecule Na2Cl2
are really different chemical species, so the distinction between physical
and chemical change becomes a bit fuzzy.
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Energetics
and equilibrium
You have probably seen chemical
reaction equations such as the "generic" one shown below:
A + B → C + D
An equation of this kind does not
imply that the reactants A and B will change entirely into the products C
and D, although in many cases this will be what appears to happen. Most
chemical reactions proceed to some inermediate point that yields a mixture of
reactants and products.
For example, if the two gases
phosphorus trichloride and chlorine are mixed together at room temprature, they
will combine until about half of them have changed into phosphorus
pentachloride:
PCl3 + Cl2 →
PCl5
At other temperatures the extent of
reaction will be smaller or greater. The result, in any case, will be an equilibrium
mixture of reactants and products.
The most important question we can
ask about any reaction is "what is the equilibrium composition"?
- If the answer is "all products and negligible quantities of reactants", then we say the reaction can takes place and that it "goes to completion ".
- If the answer is "negligible quantities of products", then we say the reaction cannot take place in the forward direction, but that the reverse reaction can occur.
- If the answer is "significant quantities of all components" (both reactans and products) are present in the equilibrium mixture, then we say the reaction is "reversible" or "incomplete".
The aspect of "change" we
are looking at here is a property of a chemical reaction, rather than of
any one substance. But if you stop to think of the huge number of possible reactions
between the more than 15 million known substances, you can see that it would be
an impossible task to measure and record the equilibrium compositions of every
possible combination.
Fortunately, we don't need to do
this. One or two directly measurable properties of the individual reactants and
products can be combined to give a number from which the equilibrium
composition at any temperature can be easily calculated. There is no need to do
an experiment!
This is very much a macroscopic view
because the properties we need to directly concern ourselves with are those of
the reactants and products. Similarly, the equilibrium composition— the measure
of the extent to which a reaction takes place— is expressed in terms of the
quantities of these substances.
Chemical
Energetics
Virtually all chemical changes
involve the uptake or release of energy, usually in the form of heat. It turns
out that these energy changes, which are the province of chemical
thermodynamics, serve as a powerful means of predicting whether or not a
given reaction can proceed, and to what extent. Moreover, all we need in order
to make this prediction is information about the energetic properties of the
reactants and products; there is no need to study the reaction itself. Because
these are bulk properties of matter, chemical thermodynamics is entirely
macroscopic in its outlook.
Dynamics:
kinetics and mechanism
The energetics of chemical change
that we discussed immediately above relate to the end result of
chemical change: the composition of the final reaction mixture, and the
quantity of heat liberated or absorbed.
The dynamics of chemical
change are concerned with how the reaction takes place:
- What has to happen to get the reaction started (which molecule gets bumped first, how hard, and from what direction?)
- Does the reaction take place in a single step, or are multiple steps and intermediate structures involved?
These details constitute what
chemists call the mechanism of the reaction. For example, the reaction
between nitric oxide and hydrogen (identified as the net reaction at the bottom
left), is believed to take place in the two steps shown here. Notice that the
nitrous oxide, N2O, is formed in the first step and consumed in the
second, so it does not appear in the net reaction equation. The N2O
is said to act as an intermediate in this reaction. Some intermediates
are unstable species, often distorted or incomplete molecules that have no
independent existence; these are known as transition states.
The microscopic side of dynamics
looks at the mechanisms of chemical reactions. This refers to a
"blow-by-blow" description of what happens when the atoms in the reacting
species re-arrange themselves into the configurations they have in the
products.
Mechanism represents the microscopic
aspect of chemical change. Mechanisms, unlike energetics, cannot be predicted
from information about the reactants and products; chemical theory has not yet
advanced to the point were we can do much more than make educated guesses. To
make matters even more complicated (or, to chemists, interesting!), the
same reaction can often proceed via different mechanisms under different
conditions.
Kinetics
Because we cannot directly watch the
molecules as they react, the best we can usually do is to infer a reaction
mechanism from experimental data, particularly that which relates to the rate
of the reaction as it is influenced by the concentrations of the reactants.
This entirely experimental area of chemical dynamics is known as kinetics.
Reaction rates, as they are called,
vary immensly: some reactions are completed in microseconds, others may take
years; many are so slow that their rates are essentially zero. To make things
even more interesting, there is no relation between reaction rates and
"tendency to react" as governed by the factors in the top half of the
above diagram; the latter can be accurately predicted from energetic data on
the substances (the properties we mentioned in the previous screen), but
reaction rates must be determined by experiment.
Catalysts
Catalysts can make dramatic changes
in rates of reactions, especially in those whose un-catalyzed rate is
essentially zero. Consider, for example, this rate data on the decomposition of
hydrogen peroxide. H2O2 is a by-product of respiration
that is poisonous to living cells which have, as a consquence, evolved a highly
efficient enzyme (a biological catalyst) that is able to destroy peroxide as
quickly as it forms. Catalysts work by enabling a reaction to proceed by an
alternative mechanism.
In some reactions, even light
can act as a catalyst. For example, the gaseous elements hydrogen and chlorine
can remain mixed together in the dark indefinitely without any sign of a
reaction, but in the sunlight they combine explosively.
In the preceding section we looked
at chemistry from a conceptual standpoint. If this can be considered a
"macroscopic" view of chemistry, what is the "microscopic"
view? It would likely be what chemists actually do. Because a thorough
exploration of this would lead us into far more detail than we can accommodate
here, we will mention only a few of the areas that have emerged as being
especially important in modern chemistry.
Separation
science
A surprisingly large part of
chemistry has to do with isolating one component from a mixture. This may occur
at any number of stages in a manufacturing process, including the very critical
steps involved in removing toxic, odiferous, or otherwise undesirable
by-products from a waste stream. But even in the research lab, a considerable
amount of effort is often devoted to separating the desired substance from the
many components of a reaction mixture, or in separating a component from a
complex mixture (for example, a drug metabolite from a urine sample) prior to
measuring the amount present.
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Distillation - separation of liquids having different boiling points.
This ancient technique (believed to have originated with Arabic
alchemists in 3500 BCE), is still one
of the most widely employed operations both in the laboratory and in
industrial processes such as oil refining.
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Solvent extraction - separation of substances based on their differing
solubilities. A common laboratory tool for isolating substances from plants
and chemical reaction mixtures. Practical uses include processing of radioactive wastes
and decaffienation of coffee beans. The separatory funnel shown here is the simplest
apparatus for liquid-liquid extraction; for solid-liquid extraction, the Soxhlet
apparatus is commonly used.
Wikipedia article
on solvent extraction
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Chromatography - This extremely versatile method depends on the tendency
of different kinds of molecules to adsorb (attach) to different surfaces as
they travel along a "column" of the adsorbent material. Just as the
progress of people walking through a shopping mall depends on how long they
spend looking in the windows they pass, those molecules that adsorb more
strongly to a material will emerge from the chromatography column more slowly
than molecules that are not so strongly adsorbed.
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Gel electrophoresis - a powerful method for separating and
"fingerprinting" macromolecules such as nucleic acids or proteins
on the basis of physcal properties such as size and electric charge.
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Identification
and assay
What do the following people have in
common?
- A plant manager deciding on whether to accept a rail tank car of vinyl chloride for manufacture into plastic pipe
- An agricultural chemist who wants to know about the vitamin content of a new vegetable hybrid
- The manager of a city water-treatment plant who needs to make sure that the carbonate content of the water is maintained high enough to prevent corrosion, but low enough to prevent scale build-up
The answer is that all depend on analytical
techniques — measurements of the nature or quantity ("assays") of
some substance of interest, sometimes at very low concentrations. A large
amount of research is devoted to finding more accurate and convenient means of
making such measurements. Many of these involve sophisticated instruments;
among the most widely used are the following:
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Spectrophotometers that examine the ways that light of various wavelengths
is absorbed, emitted, or altered by atomic and molecular species. [image
link]
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Mass spectrometers that break up molecules into fragments that can be
characterized by electrical methods. [image link]
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Instruments (NMR spectrometers)
that analyze the action of radio waves and magnetic fields on atomic nuclei
in order to examine the nature of the chemical bonds attached to a particular
kind of atom. [Image: NMR
spectrum of ethanol]
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"In the early 1900's a
chemist could analyze about 200 samples per year for the major rock-forming
elements. Today, using X-ray fluorescence spectrometry, two chemists can
perform the same type of analyses on 7,000 samples per year."
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Materials,
polymers, and nanotechnology
Materials science
attempts to relate the physical properties and performance of engineering
materials to their underlying chemical structure with the aim of developing
improved materials for various applications. The Role of Chemistry in Materials Science (a non-technical overview)
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Polymer chemistry - developing polymeric ("plastic") materials
for industrial uses. Connecting individual polymer molecules by cross-links
(red) increases the strength of the material. Thus ordinary polyethylene is a
fairly soft material with a low melting point, but the cross-linked form is
more rigid and resistent to heat. [link]
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Organic semiconductors offer a number of potential advantages over conventional
metalloid-based devices. [image link]
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Fullerenes, nanotubes and
nanowires - Fullerenes were first
identified in 1985 as products of experiments in which graphite was vaporized
using a laser, work for which R. F. Curl, Jr., R. E. Smally, and H. W. Kroto
shared the 1996 Nobel Prize in Chemistry. Fullerene research is expected to
lead to new materials, lubricants, coatings, catalysts, electro-optical
devices, and medical applications.
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Nanodevice chemistry — constructing molecular-scale assemblies for specific
tasks such as computing, producing motions, etc.
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Biosensors and biochips - the surfaces of metals and semiconductors
"decorated" with biopolymers can serve as extremely sensitive
detectors of biological substances and infectious agents.
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Biochemistry
and Molecular biology
This field covers a wide range of
studies ranging from fundamental studies on the chemistry of gene expression
and enzyme-substrate interactions to drug design. Much of the activity in this
area is directed to efforts in drug discovery.
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Drug screening began as a largely scattershot approach in which a
pathogen or a cancer cell line was screened against hundreds or thousands of
candidate substance in the hope of finding a few "leads" that might
result in a useful therapy. This field is now highly automated and usually
involves combinatorial chemistry (see below) combined with innovative
separation and assay methods.
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Drug design looks at interactions between enzymes and possible
inhibitors. Computer-modeling is an essential tool in this work.
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Proteomics - This huge field focusses on the relations between
structure and function of proteins— of which there are about 400,000
different kinds in humans. Proteomics is related to genetics in that the DNA
sequences in genes get decoded into proteins which eventually define and
regulate a particular organism.
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Chemical genomics explores the chain of events in which signalling
molecules regulate gene expression.
Image: TGFβ
receptor signal transduction pathway
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Synthesis
In its most general sense, this word
refers to any reaction that leads to the formation of a particular molecule. It
is both one of the oldest areas of chemistry and one of the most actively
persued. Some of the major threads are
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New-molecule synthesis - Chemists are always challenged to come up with
molecules containing novel features such as new shapes or unusual types of
bonds.
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Combinatorial chemistry refers to a group of largely-automated techniques for
generating tiny quantities of huge numbers of different molecules
("libraries") and then picking out those having certain desired
properties. Although it is a major drug discovery technique, it also has many
other applications.
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Green chemistry - synthetic methods that focus on reducing or eliminating
the use or release of toxic or non-biodegradable chemicals or byproducts.
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Process chemistry bridges the gap between chemical synthesis and chemical
engineering by adapting synthetic routes to the efficient, safe, and
environmentally-responsible methods for large-scale synthesis.
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Congratulations! You have just
covered all of Chemistry, condensed into one quick and painless lesson— the
world's shortest Chemistry course! Yes, we left out a lot of the details, the
most important of which it will take you a few months of happy discovery to
pick up. But if you keep in mind the global hierarchy of composition/structure,
properties of substances, and change (equilibrium and dynamics) that we have
developed in both macroscopic and microscopic views, you will find it much
easier to assemble the details as you encounter them and to see where they fit
into the bigger picture.
Make sure you thoroughly understand
the following essential ideas which have been presented above. It is especially
imortant that you know the precise meanings of all the highlighted terms in the
context of this topic.
- Distinguish beween chemistry and physics;
- Suggest ways in which the fields of engineering, economics, and geology relate to Chemistry;
- Define the following terms, and classify them as primarily microscopic or macroscopic concepts: element, atom, compound, molecule, formula, structure.
- The two underlying concepts that govern chemical change are energetics and dynamics. What aspects of chemical change does each of these areas describe?
Part 1: Particles and waves
Q1. What is a particle?
A particle is a discrete unit of matter having the attributes of mass, momentum (and thus kinetic energy) and optionally of electric charge.Q2. What is a wave?
A wave is a periodic variation of some quantity as a function of location or time. For example, the wave motion of a vibrating guitar string is defined by the displacement of the string from its center as a function of distance along the string. A sound wave consists of variations in the pressure with location.A wave is characterized by its wavelength λ (lambda) and frequency ν (nu), which are connected by the relation
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Problem example: The velocity of sound in the air is 330 m s–1.
What is the wavelength of A440 on the piano keyboard?
Solution:
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A unique property of waves is their ability to combine constructively or destructively, depending on the relative phases of the combining waves.
Q3. What is light?
Phrasing the question in this way reflects the deterministic mode of Western thought which assumes that something cannot "be" two quite different things at the same time. The short response to this question is that all we know about light (or anything else, for that matter) are the results of experiments, and that some kinds of experiments show that light behaves like particles, and that other experiments reveal light to have the properties of waves. For the moment, it is better to amend this question toQ4. What is the wave theory of light?
In the early 19th century, the English scientist Thomas Young carried out the famous two-slit experiment which demonstrated that a beam of light, when split into two beams and then recombined, will show interference effects that can only be explained by assuming that light is a wavelike disturbance. By 1820, Augustin Fresnel had put this theory on a sound mathematical basis, but the exact nature of the waves remained unclear until the 1860's when James Clerk Maxwell developed his electromagnetic theory.From the laws of electromagnetic induction that were discovered in the period 1820-1830 by Hans Christian Oersted and Michael Faraday, it was known that a moving electric charge gives rise to a magnetic field, and that a changing magnetic field can induce electric charges to move. Maxwell showed theoretically that when an electric charge is accelerated (by being made to oscillate within a piece of wire, for example), electrical energy will be lost, and an equivalent amount of energy is radiated into space, spreading out as a series of waves extending in all directions.
What is "waving" in electromagnetic radiation? According to
Maxwell, it is the strengths of the electric and magnetic fields as they travel
through space. The two fields are oriented at right angles to each other and to
the direction of travel.
As the electric field changes, it induces a magnetic field, which
then induces a new electric field, etc., allowing the wave to propagate itself
through space
These waves consist of periodic variations in the electrostatic and
electromagnetic field strengths. These variations occur at right angles to each
other. Each electrostatic component of the wave induces a magnetic component,
which then creates a new electrostatic component, so that the wave, once
formed, continues to propagate through space, essentially feeding on itself. In
one of the most brilliant mathematical developments in the history of science,
Maxwell expounded a detailed theory, and even showed that these waves should
travel at about 3E8 m s–1, a value which experimental
observations had shown corresponded to the speed of light. In 1887, the German
physicist Heinrich Hertz
demonstrated that an oscillating electric charge (in what was in essence the
world's first radio transmitting antenna) actually does produce electromagnetic
radiation just as Maxwell had predicted, and that these waves behave exactly
like light.Part 2: Quantum theory of light
Q5. How did the quantum theory of light come about?
It did not arise from any attempt to explain the behavior of light itself; by 1890 it was generally accepted that the electromagnetic theory could explain all of the properties of light that were then known.Certain aspects of the interaction between light and matter that were observed during the next decade proved rather troublesome, however. The relation between the temperature of an object and the peak wavelength emitted by it was established empirically by Wilhelm Wien in 1893. This put on a quantitative basis what everyone knows: the hotter the object, the "bluer" the light it emits.
Q6. What is black body radiation?
All objects above the temperature of absolute zero emit electromagnetic radiation consisting of a broad range of wavelengths described by a distribution curve whose peak wavelength l at absolute temperature T for a "perfect radiator" known as a black body is given by Wien's law
At ordinary temperatures this radiation is entirely in the infrared
region of the spectrum, but as the temperature rises above about 1000K, more
energy is emitted in the visible wavelength region and the object begins to
glow, first with red light, and then shifting toward the blue as the
temperature is increased.
This type of radiation has two important characteristics. First,
the spectrum is a continuous one, meaning that all wavelengths are emitted,
although with intensities that vary smoothly with wavelength. The other curious
property of black body radiation is that it is independent of the composition
of the object; all that is important is the temperature.
Q7. How did black body radiation lead to quantum physics?
Black body radiation, like all electromagnetic radiation, must originate from oscillations of electric charges which in this case were assumed to be the electrons within the atoms of an object acting somewhat as miniature Hertzian oscillators. It was presumed that since all wavelengths seemed to be present in the continuous spectrum of a glowing body, these tiny oscillators could send or receive any portion of their total energy. However, all attempts to predict the actual shape of the emission spectrum of a glowing object on the basis of classical physical theory proved futile.In 1900, the great German physicist Max Planck (who earlier in the same year had worked out an empirical formula giving the detailed shape of the black body emission spectrum) showed that the shape of the observed spectrum could be exactly predicted if the energies emitted or absorbed by each oscillator were restricted to integral values of hν, where ν ("nu") is the frequency and h is a constant 6.626E–34 J s which we now know as Planck's Constant. The allowable energies of each oscillator are quantized, but the emission spectrum of the body remains continuous because of differences in frequency among the uncountable numbers of oscillators it contains.This modification of classical theory, the first use of the quantum concept, was as unprecedented as it was simple, and it set the stage for the development of modern quantum physics.
Q8. What is the photoelectric effect?
Shortly after J.J. Thompson's experiments led to the identification of the elementary charged particles we now know as electrons, it was discovered that the illumination of a metallic surface by light can cause electrons to be emitted from the surface. This phenomenon, the photoelectric effect, is studied by illuminating one of two metal plates in an evacuated tube. The kinetic energy of the photoelectrons causes them to move to the opposite electrode, thus completing the circuit and producing a measurable current. However, if an opposing potential (the retarding potential) is imposed between the two plates, the kinetic energy can be reduced to zero so that the electron current is stopped. By observing the value of the retarding potential Vr, the kinetic energy of the photoelectrons can be calculated from the electron charge e, its mass m and the frequency of the incident light:
These two diagrams are taken from a Web
page by Joseph Alward of the University of the Pacific.
The plot at the right shows how the kinetic energy of the
photoelectrons falls to zero at the critical wavelength corresponding to
frequency f0.
Q9. What peculiarity of the photoelectric effect led to the photon?
Although the number of electrons ejected from the metal surface per second depends on the intensity of the light, as expected, the kinetic energies of these electrons (as determined by measuring the retarding potential needed to stop them) does not, and this was definitely not expected. Just as a more intense physical disturbance will produce higher energy waves on the surface of the ocean, it was supposed that a more intense light beam would confer greater energy on the photoelectrons. But what was found, to everyone's surprise, is that the photoelectron energy is controlled by the wavelength of the light, and that there is a critical wavelength below which no photoelectrons are emitted at all.Albert Einstein quickly saw that if the kinetic energy of the photoelectrons depends on the wavelength of the light, then so must its energy. Further, if Planck was correct in supposing that energy must be exchanged in packets restricted to certain values, then light must similarly be organized into energy packets. But a light ray consists of electric and magnetic fields that spread out in a uniform, continuous manner; how can a continuously-varying wave front exchange energy in discrete amounts? Einstein's answer was that the energy contained in each packet of the light must be concentrated into a tiny region of the wave front. This is tantamount to saying that light has the nature of a quantized particle whose energy is given by the product of Planck's constant and the frequency:
Q10. Where does relativity come in?
Einstein's special theory of relativity arose from his attempt to understand why the laws of physics that describe the current induced in a fixed conductor when a magnet moves past it are not formulated in the same way as the ones that describe the magnetic field produced by a moving conductor. The details of this development are not relevant to our immediate purpose, but some of the conclusions that this line of thinking led to very definitely are. Einstein showed that the velocity of light, unlike that of a material body, has the same value no matter what velocity the observer has. Further, the mass of any material object, which had previously been regarded as an absolute, is itself a function of the velocity of the body relative to that of the observer (hence "relativity"), the relation being given byAccording to this formula, the mass of an object increases without limit as the velocity approaches that of light. Where does the increased mass come from? Einstein's answer was that the increased mass is that of the kinetic energy of the object; that is, energy itself has mass, so that mass and energy are equivalent according to the famous formula
Q11. Can the mass-less photon have momentum?
Although the photon has no rest mass, its energy, given by , confers upon it an effective mass ofQ12. If waves can be particles, can particles be waves?
In 1924, the French physicist Louis deBroglie proposed (in his doctoral thesis) that just as light possesses particle-like properties, so should particles of matter exhibit a wave-like character. Within two years this hypothesis had been confirmed experimentally by observing the diffraction (a wave interference effect) produced by a beam of electrons as they were scattered by the row of atoms at the surface of a metal.deBroglie showed that the wavelength of a particle is inversely proportional to its momentum:
Q13. Exactly what is it that is "waving"?
We pointed out earlier that a wave is a change that varies with location in a periodic, repeating way. What kind of a change do the crests and hollows of a "matter wave" trace out? The answer is that the wave represents the value of a quantity whose square is a measure of the probability of finding the particle in that particular location. In other words, what is "waving" is the value of a mathematical probability function.Q14. What is the uncertainty principle?
In 1927, Werner Heisenberg proposed that certain pairs of properties of a particle cannot simultaneously have exact values. In particular, the position and the momentum of a particle have associated with them uncertainties x and p given byQ15. Is the uncertainty principle consistent with particle waves?
Yes; either one really implies the other. Consider the following two limiting cases: · A particle whose velocity is known to within a very small uncertainty will have a sharply-defined energy (because its kinetic energy is known) which can be represented by a probability wave having a single, sharply-defined frequency. A "monochromatic" wave of this kind must extend infinitely in space:To help you see how waveforms of different wavelength combine, two such combinations are shown below:
Q16. Are they particles or are they waves?
Suppose we direct a beam of photons (or electrons; the experiment works with both) toward a piece of metal having a narrow opening. On the other side there are two more openings, or slits. Finally the particles impinge on a photographic plate or some other recording device. Taking into account their wavelike character, we would expect the probability waves to produce an interference pattern of the kind that is well known for sound and light waves, and this is exactly what is observed; the plate records a series of alternating dark and light bands, thus demonstrating beyond doubt that electrons and light have the character of waves.Now let us reduce the intensity of the light so that only one photon at a time passes through the apparatus (it is experimentally possible to count single photons, so this is a practical experiment). Each photon passes through the first slit, and then through one or the other of the second set of slits, eventually striking the photographic film where it creates a tiny dot. If we develop the film after a sufficient number of photons have passed through, we find the very same interference pattern we obtained previously.
There is something strange here. Each photon, acting as a particle, must pass through one or the other of the pair of slits, so we would expect to get only two groups of spots on the film, each opposite one of the two slits. Instead, it appears that the each particle, on passing through one slit, "knows" about the other, and adjusts its final trajectory so as to build up a wavelike interference pattern.
It gets even stranger: suppose that we set up a detector to determine which slit a photon is heading for, and then block off the other slit with a shutter. We find that the photon sails straight through the open slit and onto the film without trying to create any kind of an interference pattern. Apparently, any attempt to observe the photon as a discrete particle causes it to behave like one.
The only conclusion possible is that quantum particles have no well defined paths; each photon (or electron) seems to have an infinity of paths which thread their way through space, seeking out and collecting information about all possible routes, and then adjusting its behavior so that its final trajectory, when combined with that of others, produces the same overall effect that we would see from a train of waves of wavelength λ= h/mv.
Click here for a simulation of this
experiment (may not work with some browsers.)
See here for another
description of the two-slit experiment.
Wikipedia page
on the two-slit experiment
Part 3: Electrons in atoms
At first it seemed a little speck, and then it seemed a
mist;
it moved and moved, and took at last
a certain shape, I wist.
it moved and moved, and took at last
a certain shape, I wist.
(Samuel Taylor Coleridge;
The Rime of the Ancient Mariner)
The Rime of the Ancient Mariner)
Q17. What are line spectra?
We have already seen that a glowing body (or actually, any body whose temperature is above absolute zero) emits and absorbs radiation of all wavelength in a continuous spectrum. In striking contrast is the spectrum of light produced when certain substances are volatilized in a flame, or when an electric discharge is passed through a tube containing gaseous atoms of an element. The light emitted by such sources consists entirely of discrete wavelengths. This kind of emission is known as a discrete spectrum or line spectrum (the "lines" that appear on photographic images of the spectrum are really images of the slit through which the light passes before being dispersed by the prism in the spectrograph).Every element has its own line spectrum which serves as a sensitive and useful tool for detecting the presence and relative abundance of the element, not only in terrestrial samples but also in stars. (As a matter of fact, the element helium was discovered in the sun, through its line spectrum, before it had been found on Earth.) In some elements, most of the energy in the visible part of the emission spectrum is concentrated into just a few lines, giving their light characteristic colors: yellow-orange for sodium, blue-green for mercury (these are commonly seen in street lights) and orange for neon.
Line spectra were well known early in the 19th century, and were widely used for the analysis of ores and metals. The German spectroscopist R.W. Bunsen, now famous for his gas burner, was then best known for discovering two new elements, rubidium and cesium, from the line spectrum he obtained from samples of mineral spring waters.
Q18. How are line spectra organized?
Until 1885, line spectra were little more than "fingerprints" of the elements; extremely useful in themselves, but incapable of revealing any more than the identify of the individual atoms from which they arise. In that year a Swiss school teacher named Johann Balmer published a formula that related the wavelengths of the four known lines in the emission spectrum of hydrogen in a simple way. Balmer's formula was not based on theory; it was probably a case of cut-and-try, but it worked: he was able to predict the wavelength of a fifth, yet-to-be discovered emission line of hydrogen, and as spectroscopic and astronomical techniques improved (the only way of observing highly excited hydrogen atoms at the time was to observe the solar spectrum during an eclipse), a total of 35 lines were discovered, all having wavelengths given by the formula which we write in the modern manner asIt was soon discovered that by replacing m with integers other than 2, other series of hydrogen lines could be accounted for. These series, which span the wavelength region from the ultraviolet through infrared, are named after their discoverers.
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name of series
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when discovered
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value of m
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Lyman
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1906-14
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1
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Balmer
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1885
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2
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Paschen
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1908
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3
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Brackett
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1922
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4
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Pfund
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1924
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5
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Q19. How large can n be?
There is no limit; values in the hundreds have been observed, although doing so is very difficult because of the increasingly close spacing of successive levels as n becomes large. Atoms excited to very high values of n are said to be in Rydberg states.Q20. Why do line spectra become continuous at short wavelengths?
As n becomes larger, the spacing between neighboring levels diminishes and the discrete lines merge into a continuum. This can mean only one thing: the energy levels converge as n approaches infinity. This convergence limit corresponds to the energy required to completely remove the electron from the atom; it is the ionization energy.Q21. What were the problems with the planetary model of the atom?
Rutherford's demonstration that the mass and the positive charge of the atom is mostly concentrated in a very tiny region called the nucleus forced the question of just how the electrons are disposed outside the nucleus. By analogy with the solar system, a planetary model was suggested: if the electrons were orbiting the nucleus, there would be a centrifugal force that could oppose the electrostatic attraction and thus keep the electrons from falling into the nucleus. This of course is similar to the way in which the centrifugal force produced by an orbiting planet exactly balances the force due to its gravitational attraction to the sun.
The planetary model suffers from one fatal weakness: electrons,
unlike planets, are electrically charged. An electric charge revolving in an
orbit is continually undergoing a change of direction, that is, acceleration.
It has been well known since the time of Hertz that an accelerating electric
charge radiates energy. We would therefore expect all atoms to act as miniature
radio stations. Even worse, conservation of energy requires that any energy
that is radiated must be at the expense of the kinetic energy of the orbital
motion of the electron. Thus the electron would slow down, reducing the
centrifugal force and allowing the electron to spiral closer and closer to the
nucleus, eventually falling into it. In short, no atom that operates according
to the planetary model would last long enough for us to talk about it.
As if this were not enough, the planetary model was totally unable to
explain any of the observed properties of atoms, including their line spectra. Q22. How did Bohr's theory save the planetary model... for a while?
Niels Bohr was born in the same year (1885) that Balmer published his formula for the line spectrum of hydrogen. Beginning in 1913, the brilliant Danish physicist published a series of papers that would ultimately derive Balmer's formula from first principles.Bohr's first task was to explain why the orbiting electron does not radiate energy as it moves around the nucleus. This energy loss, if it were to occur at all, would do so gradually and smoothly. But Planck had shown that black body radiation could only be explained if energy changes were limited to jumps instead of gradual changes. If this were a universal characteristic of energy- that is, if all energy changes were quantized, then very small changes in energy would be impossible, so that the electron would in effect be "locked in" to its orbit.
From this, Bohr went on to propose that there are certain stable orbits in which the electron can exist without radiating and thus without falling into a "death spiral". This supposition was a daring one at the time because it was inconsistent with classical physics, and the theory which would eventually lend it support would not come along until the work of de Broglie and Heisenberg more than ten years later.
Since Planck's quanta came in multiples of h, Bohr restricted his allowed orbits to those in which the product of the radius r and the momentum of the electron mv (which has the same units as h, J–s) are integral multiples of h:
2πrmv = nh (n
= 1,2,3, . .)
Each orbit corresponds to a different energy, with the electron normally
occupying the one having the lowest energy, which would be the innermost orbit
of the hydrogen atom.Taking the lead from Einstein's explanation of the photoelectric effect, Bohr assumed that each spectral line emitted by an atom that has been excited by absorption of energy from an electrical discharge or a flame represents a change in energy given by ΔE = hn = hc/λ, the energy lost when the electron falls from a higher orbit (value of n) into a lower one.
Finally, as a crowning triumph, Bohr derived an expression giving the radius of the nth orbit for the electron in hydrogen as
Q23. What were the main problems with Bohr's theory?
There were two kinds of difficulties. First, there was the practical limitation that it only works for atoms that have one electron-- that is, for H, He+, Li2+, etc. The second problem was that Bohr was unable to provide any theoretical justification for his assumption that electrons in orbits described by the preceding equation would not lose energy by radiation. This reflects the fundamental underlying difficulty: because de Broglie's picture of matter waves would not come until a decade later, Bohr had to regard the electron as a classical particle traversing a definite orbital path.Q24. How did the wave picture of the electron save Bohr's theory?
Once it became apparent that the electron must have a wavelike character, things began to fall into place. The possible states of an electron confined to a fixed space are in many ways analogous to the allowed states of a vibrating guitar string. These states are described as standing waves that must possess integral numbers of nodes. The states of vibration of the string are described by a series of integral numbers n = 1,2,... which we call the fundamental, first overtone, second overtone, etc. The energy of vibration is proportional to n2. Each mode of vibration contains one more complete wave than the one below it.In exactly the same way, the mathematical function that defines the probability of finding the electron at any given location within a confined space possesses n peaks and corresponds to states in which the energy is proportional to n2.
The electron in a hydrogen atom is bound to the nucleus by its spherically symmetrical electrostatic charge, and should therefore exhibit a similar kind of wave behavior. This is most easily visualized in a two-dimensional cross section that corresponds to the conventional electron orbit. But if the particle picture is replaced by de Broglie's probability wave, this wave must follow a circular path, and- most important of all- its wavelength (and consequently its energy) is restricted to integral multiples n = 1,2,.. of the circumference 2πr = nλ.
for otherwise the wave would collapse owing to self-interference. That is, the energy of the electron must be quantized; what Bohr had taken as a daring but arbitrary assumption was now seen as a fundamental requirement. Indeed the above equation can be derived very simply by combining Bohr's quantum condition 2πrmv = nh with the expression mv = h/λ for the deBroglie wavelength of a particle.
Q25. What is an orbital?
Because the classical view of an electron as a localizable particle is now seen to be untenable, so is the concept of a definite trajectory, or "orbit". Instead, we now use the word orbital to describe the state of existence of an electron. An orbital is really no more than a mathematical function describing the standing wave that gives the probability of the electron manifesting itself at any given location in space. More commonly (and loosely) we use the word to describe the region of space in which an electron is likely to be found. Each kind of orbital is characterized by a set of quantum numbers n, l, and m These relate, respectively, to the average distance of the electron from the nucleus, to the shape of the orbital, and to its orientation in space.Q26. If the electron cannot be localized, can it be moving?
In its lowest state in the hydrogen atom (in which l=0) the electron has zero angular momentum, so electrons in s orbitals are not in motion. In orbitals for which l>0 the electron does have an effective angular momentum, and since the electron also has a definite rest mass me = 9.11E31 kg, it must possess an effective velocity. Its value can be estimated from the Uncertainty Principle; if the volume in which the electron is confined is about 10–10 m, then the uncertainty in its momentum is at least h/(1010) = 6.6E–24 kg m s–1, which implies a velocity of around 107 m s–1, or almost one-tenth the velocity of light.The stronger the electrostatic force of attraction by the nucleus, the faster the effective electron velocity. In fact, the innermost electrons of the heavier elements have effective velocities so high that relativistic effects set in; that is, the effective mass of the electron significantly exceeds its rest mass. This has direct chemical effects; it is the cause, for example, of the low melting point of metallic mercury and of the color of gold.
Q27. Why does the electron not fall into the nucleus?
The following is a
very brief answer; for a more complete explanation, please see
"How the battle of the infinities saves the electron from its death spiral".
The negatively-charged electron is attracted to the positive charge of the
nucleus. What prevents it from falling in? This question can be answered in
various ways at various levels. All start with the statement that the electron,
being a quantum particle, has a dual character and cannot be treated solely by
the laws of Newtonian mechanics."How the battle of the infinities saves the electron from its death spiral".
We saw above that in its wavelike guise, the electron exists as a standing wave which must circle the nucleus at a sufficient distance to allow at least one wavelength to fit on its circumference. This means that the smaller the radius of the circle, the shorter must be the wavelength of the electron, and thus the higher the energy. Thus it ends up "costing" the electron energy if it gets too close to the nucleus. The normal orbital radius represents the balance between the electrostatic force trying to pull the electron in, and what we might call the "confinement energy" that opposes the electrostatic energy. This confinement energy can be related to both the particle and wave character of the electron.
If the electron as a particle were to approach the nucleus, the uncertainty in its position would become so small (owing to the very small volume of space close to the nucleus) that the momentum, and therefore the energy, would have to become very large. The electron would, in effect, be "kicked out" of the nuclear region by the confinement energy.
The standing-wave patterns of an electron in a box can be calculated quite easily. For a spherical enclosure of diameter d, the energy is given by
in which n = 1,2,3. etc.
Q28. What is electron spin?
Each electron in an atom has associated with it a magnetic field whose direction is quantized; there are only two possible values that point in opposite directions. We usually refer to these as "up" and "down", but the actual directions are parallel and antiparallel to the local magnetic field associated with the orbital motion of the electron.The term spin implies that this magnetic moment is produced by the electron charge as the electron rotates about its own axis. Although this conveys a vivid mental picture of the source of the magnetism, the electron is not an extended body and its rotation is meaningless. Electron spin has no classical counterpart and no simple explanation; the magnetic moment is a consequence of relativistic shifts in local space and time due to the high effective velocity of the electron in the atom. This effect was predicted theoretically by P.A.M. Dirac in 1928.
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