To explain the structure
of the atom, the Danish physicist Niels Bohr developed in 1913
a hypothesis known as the Bohr theory of the atom. He assumed
that electrons are arranged in definite shells, or quantum
levels, at a considerable distance from the nucleus.
The arrangement of these
electrons is called the electron configuration. The number
of such electrons equals the atomic number of the atom;
hydrogen has a single orbital electron, helium has 2, and uranium
The electron shells are
built up in a regular fashion from a first shell to a total
of seven shells, each of which has an upper limit to the number
of electrons that it can accommodate. The first shell is complete
with two electrons, the second can hold up to eight electrons,
and successive shells hold still larger numbers.
electrons, those which are outermost or added last to the atom's
structure, determine the chemical behavior of the atom.
The inert, or noble, gases
(helium, neon, argon, krypton, xenon, and radon) all have completely
filled outer shells. They do not enter into chemical combinations
in nature, although the three heaviest inert gases (krypton,
xenon, and radon) have formed chemical compounds in the laboratory.
On the other hand, the
outermost shells of such elements as lithium, sodium, and potassium
contain only one electron. These elements combine readily
with other elements (transferring their outermost electrons
to them) to form a great many chemical compounds.
Atomic shells do not
necessarily fill up with electrons in consecutive order. The
electrons of the first 18 elements in the periodic table are added
in a regular manner, each shell being filled to a designated limit
before a new shell is started. Beginning with the 19th element,
the outermost electron starts a new shell before the previous
shell is completely filled. A regularity is still maintained,
however, as electrons fill successive shells in a repetitious
back-and-forth pattern. The result is the regular repetition of
chemical properties for atoms of increasing atomic weight that
corresponds to the arrangement of the elements in the
It is convenient to
visualize the electrons moving about the nucleus of an atom much
as if they were planets moving about the sun. This view
is much more precise than that held by contemporary physicists,
however. It is now known that it is impossible to pinpoint
the precise position of an electron in the atom's space without
disturbing its predicted location at some future time.
is resolved by attributing to the atom a cloudlike form,
in which the electron's position is defined in terms of the probability
of finding it at some distance from the nucleus. This rather fuzzy
schematic conception of the atom may be reconciled with the solar-system
model by noting that in the tiny space of the atom the electron,
which makes many billions of orbits around the nucleus in a single
second, is everywhere at once.
The cloud view thus gives
a form to the atom that is not supplied by a solar-system model