The Atom

An atom is the smallest unit of matter that is
recognizable as a chemical ELEMENT. Atoms of
different elements may also combine into systems
called MOLECULES, which are the smallest units
of chemical COMPOUNDS. In all these ordinary
processes, atoms may be considered as the
ancient Greeks imagined them to be: the ultimate
building blocks of matter. When stronger forces
are applied to atoms, however, the atoms may
break up into smaller parts. Thus atoms are
actually composites and not units, and have a
complex inner structure of their own. By studying
the processes in which atoms break up, scientists
in the 20th century have come to understand many
details of the inner structure of atoms. The size of
a typical atom is only about 10 (-10th) meters. A
cubic centimeter of solid matter contains
something like 10 (24th) atoms. Atoms cannot be
seen using optical microscopes, because they are
much smaller than the wavelengths of visible light.
By using more advanced imaging techniques such
as electron microscopes, scanning tunneling
microscopes, and atomic force microscopes,
however, scientists have been able to produce
images in which the sites of individual atoms can
be identified. EARLY ATOMIC THEORIES The
first recorded speculations that MATTER
consisted of atoms are found in the works of the
Greek philosophers LEUCIPPUS and
DEMOCRITUS. The essence of their views is
that all phenomena are to be understood in terms
of the motions, through empty space, of a large
number of tiny and indivisible bodies. (The name
"atom" comes from the Greek words atomos, for
"indivisible.") According to Democritus, these
bodies differ from one another in shape and size,
and the observed variety of substances derives
from these differences in the atoms composing
them. Greek atomic theory was not an attempt to
account for specific details of physical phenomena.
It was instead a philosophical response to the
question of how change can occur in nature. Little
effort was made to make atomic theory
quantitative--that is, to develop it as a scientific
hypothesis for the study of matter. Greek atomism,
however, did introduce the valuable concept that
the nature of everyday things was to be
understood in terms of an invisible substructure of
objects with unfamiliar properties. Democritus
stated this especially clearly in one of the few
sayings of his that has been preserved: "Color
exists by convention, sweet by convention, bitter
by convention, in reality nothing exists but atoms
and the void." Although adopted and extended by
such later ancient thinkers as EPICURUS and
LUCRETIUS, Greek atomic theory had strong
competition from other views of the nature of
matter. One such view was the four-element
theory of EMPEDOCLES. These alternative
views, championed by ARISTOTLE among
others, were also motivated more by a desire to
answer philosophical questions than by a wish to
explain scientific phenomena. ORIGINS OF
MODERN ATOMISM When interest in science
revived in Europe in the 16th and 17th centuries,
enough was known about Greek atomism to form
the basis for further thought. Among those who
revived the atomic theory were Pierre
GASSENDI, Robert BOYLE, and especially
Isaac NEWTON. The latter part of Newton\'s
book Optiks is a series of detailed speculations on
the atomic nature of matter and light, indicating
how some of matter\'s properties are to be
understood in terms of atoms. In the 19th century,
two independent lines of reasoning strengthened
the belief of most scientists, by then, in the atomic
theory. Both approaches also began to reveal
some quantitative properties of atoms. One
approach, pioneered by John DALTON, involved
chemical phenomena. The other, involving the
behavior of gases, was carried out by physicists
such as Rudolph CLAUSIUS and James Clerk
MAXWELL. Dalton\'s main step forward was his
introduction of ATOMIC WEIGHTS. Dalton
studied the elements then known and analyzed the
data of their reactions with one another. He
discovered the law of multiple proportions, which
states that when several distinct reactions take
place among the same elements, the quantities that
enter the reactions are always in the proportions of
simple integers--that is, 1 to 1, 2 to 1, 2 to 3, and
so on. From this came the concept that such
reacting quantities contain equal numbers of atoms
and are therefore proportional to the masses of
individual atoms. Dalton gave the lightest known
element, hydrogen, an atomic weight of 1, and
developed comparative atomic weights for the
other known elements accordingly. The study of
gases in terms of atomic theory was begun by
Daniel BERNOULLI in the 18th century.
Bernoulli showed that the pressure exerted by a
gas came about as the result of collisions of the
atoms of the gas with the walls of its container. In
1811, Amadeo AVOGADRO suggested that
equal volumes of different gases, under the same
conditions of pressure and temperature, contain
equal numbers of atoms. The number of atoms in a
mass of gas equal to one gram atomic weight--a
quantity of an element, in grams,