Atoms
All matter is composed of atoms, which are very tiny objects. We cannot
see them because waves of light wash over them like ocean waves over a
pebble. An atom is about as big in comparison to a human cell, which can
be clearly seen under an ordinary microscope, as a human cell is in
comparison to a billiard ball. Somewhat more precisely, a hundred
million atoms laid side by side would be about an inch in length.
Despite its Greek name, which means indivisible, the atom is made up of
parts. It consists of a central nucleus, which carries a positive
electrical charge, around which one or more negatively charged electrons
are distributed. One frequently hears of the electrons revolving in
orbits around the nucleus, somewhat as the planets revolve around the
sun in our own solar system. This is not quite a correct picture,
however. For one thing the electrons are more elusive than the planets.
They do not revolve in definite orbits as the planets do. Also the
orbits are more delicate. One would destroy the atom by the attempt to
find out precisely what the electron orbits are.
[Illustration: This is how an atom does _not_ look. The electrons do
not move along well-defined paths. It is more difficult to convey
the idea of an atom by a picture than it is to make a drawing of
last night’s dream.]
The planets do not fly away from the sun because of the gravitational
attraction which the sun exerts. The electrons and the nucleus, however,
are held together because positive and negative electrical charges
attract each other. The gravitational attraction between the electrons
and the nucleus is incredibly weak compared to the electrical
attraction.
Most of the atom’s weight comes from its nucleus. Even the lightest
known nucleus weighs about 1840 times as much as an electron. In spite
of this, the nucleus occupies only a tiny portion of the total volume of
the atom. In fact, the nucleus is about as big in comparison to the
whole atom as the atom is in comparison to the human cell. Twenty
thousand nuclei laid side by side would be about equal in length to the
diameter of the atom. If matter were composed of nothing but nuclei
densely packed together, an object the size of a penny would weigh
approximately forty million tons.
Later we are going to see that the size of the nucleus has a great
effect upon the ways in which nuclei react with each other. For that
very reason the size of the nucleus is a well-defined measurable
quantity. It is much harder to say precisely what one means by the size
of the electron. It seems acceptable to say that it is somewhat less
than the size of the average nucleus. In any case it is certain that
both the electrons and the nucleus are small compared to the size of the
whole atom. Consequently, the atom must consist mostly of empty space.
This means, of course, that when you look at solid matter, what is
before your eyes is empty space with a slight addition of real
substance. What lends strength to solids is the interplay of electric
attractions and repulsions inside atoms and between atoms.
When a charged particle, such as an electron or a nucleus, happens to
move through solid matter, it is constantly acted on by large electric
forces. To such a particle matter does not seem to be very transparent.
But if there were such a thing as an electrically neutral particle,
comparable in size to the nucleus, it would be able to move around
freely inside matter, without experiencing electric forces, and only now
and again bumping into a nucleus or maybe an electron. As a matter of
fact, there is such a particle and it can pass right through an inch or
two of solid matter without bumping into anything. Later on in this book
we shall be very interested in this particle, which is called a neutron.
Although the electrons and the nucleus are charged particles, the atom
as a whole is electrically neutral; this means that the positive charge
of the nucleus must be equal in magnitude to the total charge of all the
negative electrons. All electrons have precisely the same charge, which
is the smallest charge that has ever been observed. What is particularly
strange and not yet explained is the fact that all other charges are as
big as the electron charge, or twice as big, or three times as big, or a
million times as big. But we never find a charge which, expressed in
terms of the electron charge, is fractional. No object ever carries
three and a half electron charges. The electron charge therefore may be
used conveniently as a standard unit of charge.
Every atom can be distinguished by the charge of its nucleus. The
simplest atom one can imagine would clearly be one with a single
electron revolving around a nucleus having one unit of positive charge.
Such an atom exists and is called hydrogen. An atom with a nucleus of
charge two and two electrons revolving around it, is called helium;
three, lithium ... six, seven, eight; carbon, nitrogen, oxygen ... 92,
uranium. Atoms with almost all charges from one to 92 are found in
nature, and practically none above 92 are found. Some odd charges—43,
61, 85, and 87—are missing. The reason for these missing atoms is
connected with the properties of the nucleus. The nucleus will soon
become our main object of interest.
The most surprising fact about atoms is their similarity, indeed their
identical behavior. If two atoms have the same kind of nucleus and have
the same number of electrons revolving around these nuclei, then these
two atoms are apt to be encountered in a condition which is most
precisely the same for the two. One could imagine that the various
component parts of the atom would be arranged in different ways and
found in different states of motion, in a variety without limit. Whence
the complete similarity? The answer to this question is not only most
surprising, but it is even in apparent contradiction to common sense.
For this very reason it is difficult to explain. The hardest things to
understand are not those which are complicated but those which are
unexpected.
Fortunately for our purpose we need not go into this more intricate
portion of atomic physics. It is sufficient to say that there is one
arrangement or pattern of motion of the electrons which is preferred and
which leads to the greatest stability of the atom. If the electrons are
in this particular state of motion, which is called the ground state,
they have less energy than they would have if they were in any other
state of motion. There are other less stable, but not less sharply
defined, states of atoms which we call “excited” states. When an atom is
in such an excited state, it tends to be unstable and tries to get into
the ground state as soon as possible. Since the ground state contains
less energy than any other state, the atom must release energy in the
process of adjustment. The released energy manifests itself in the form
of electromagnetic radiation—often as a little pulse of visible light.
The color of this light depends upon the amount of energy released,
going progressively through the rainbow from red toward blue as the
amount of energy increases.
There are very few states in which the excitation energy is small. But
of strongly excited states there is a great abundance. In the region of
this high excitation small additional changes are possible. Thus we
approach a situation more in accordance with experience and common
sense: the pattern of motion can be changed by any small amount.
The description we have just given is of course incomplete. We must
avoid here the crucial questions why only some patterns of motion are
possible, why one lowest level is stable and why the electrons never
descend into decreasing states of energy, following the attraction of
the nucleus. At the same time one should emphasize that a complete
explanation of these facts has been given. This explanation makes
precise predictions about many of the properties of matter, and we can
have complete confidence that, but for the involved mathematical
procedure, all ordinary properties of materials could be precisely
predicted. The atom has been explained as completely as Newton has
explained the motion of planets.
To form an idea what an atom is or why two atoms of, let us say,
hydrogen are precisely the same, it is not necessary to search for
intricate reasons or deep meanings. Two atoms of the same kind are alike
as two pawns are for the chess player, except for one little point: in
the case of the pawns we do not _care_ about the difference; in the case
of the atoms there _is_ no difference. This is a simple statement and it
honestly describes a simple situation. The beauty of science is due to
the fact that the correct answers to our most interesting questions have
turned out to be surprising by their simplicity.
In order to understand an atom one must consider the distribution of
electrons around one nucleus. In order to understand a molecule one has
to consider the distribution of electrons around two or more nuclei. The
_chemical_ behavior of an atom is the manner in which it interacts with
other atoms, and that means the precise way in which the electrons
rearrange themselves when two or more atoms approach each other. The
interaction between atoms occurs mainly between their outermost
electrons. It may happen that two quite different atoms, containing
nuclei of different charges and different numbers of electrons, may
nevertheless be similar in the structure of their outermost electrons.
In this case the two atoms exhibit similar chemical properties. Examples
are lithium with charge 3 and sodium with charge 11; also helium, charge
2 and neon, charge 10. A most important example for our purpose is the
set of three chemically similar atoms: calcium, charge 20, strontium,
charge 38; and radium, charge 88.
When two or more atoms approach each other, whether they are similar or
different, their electrons—particularly the outermost ones—find new
states of motion instead of those that were available to them when there
was only one nucleus in the vicinity. It may now happen that amongst
these new states of motion there are some that are even more stable than
the state of the separated atoms. In this event the atoms will tend to
stick together, and the electrons will adopt whatever new state of
motion now corresponds to maximum stability. The composite system of the
atoms is called a molecule, and its state of maximum stability, the
ground state of the molecule.
There are atoms of particularly great stability which cannot increase
their stability by combining with other atoms. Examples are helium,
neon, and argon. These atoms tend to remain single, retain their
independent motion in a rather “permanent” gaseous state, and are
generally unsociable. They are called therefore the noble gases.[1]
An especially simple example of the formation of a molecule is the
combining of sodium and chlorine to form ordinary table salt. The sodium
atom happens to have a rather loosely bound outer electron. The chlorine
atom possesses a convenient niche for an extra electron. Consequently
the energy spent in prying the outer electron loose from the sodium atom
is largely repaid by adding it to the chlorine atom. The remaining
sodium “atom,” deprived of one of its electrons, now has a net positive
charge.[2] The chlorine “atom” with its extra electron has a net
negative charge. The two “atoms” therefore attract each other to make a
molecule of sodium chloride. Actually matter will continue to aggregate.
A great number of positive sodium “atoms” and negative chlorine “atoms”
will arrange themselves into a beautiful and regular lattice which is
the sodium chloride crystal.
The simplest molecule which does not tend to grow into a bigger
aggregate is made up of two hydrogen atoms. Around two hydrogen nuclei a
particularly stable pattern of two electrons can be formed. Because of
this fact hydrogen atoms associate pairwise so that this pattern should
become possible.
The ways in which atoms can be joined are incredibly manifold. They can
form metals in which the outer electrons roam freely and carry electric
currents with the greatest of ease. They can form liquids in which atoms
or molecules are tied together in a loose and disorderly fashion. They
can move independently making occasional encounters, which is what
happens in a gas. And they can form long spiraling molecules where
groups of atoms are strung together without an apparent simple order,
but in a way which is somehow related to the processes of life.
[Illustration: Arrangement of sodium and chlorine “atoms” in a
crystal of common salt.]
We all know in how many forms matter can appear and how changeable these
forms are. That the stone and the spray, the air and an insect, and even
the human brain should be composed of the same few kinds of atoms, and
that these atoms should be subject to laws which are subtle and simple
and precisely described—this certainly is the most remarkable fact that
we have learned since Newton proved that the same science applies to the
earth and in the heavens.