The region around the nucleus of an atom (or,
in a molecule, around several nuclei) in
which an electron is most likely to be found.
According to quantum theory, the position of
an electron is uncertain; it may be found at
any point. However, it is more likely to be
found in some places than in others, and
these make up the orbital. An atom or
molecule has numerous orbitals, each of which
has a fixed size and shape. An orbital is
characterized by three numbers, called
quantum numbers, representing its energy (and
hence size), its angular momentum (and hence
shape), and its orientation. Each orbital can
be occupied by one or two electrons (by two
only if their spins are aligned in opposite
directions).