The most important properties of atomic and molecular structure may be
exemplified using a simplified picture of an
atom that is called the Bohr Model. This model was proposed by Niels Bohr in
1915; it is not completely correct, but
it has many features that are approximately correct and it is sufficient for
much of our discussion.
The correct theory
of the atom is called quantum mechanics; the Bohr Model is an approximation to
quantum mechanics that has the
virtue of being much simpler.
(Here is a
more realistic discussion of what atomic orbitals look like in quantum
mechanics.)