The Atom

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