energy of two hydrogen atoms can be represented in a diagram such
as that above. The horizontal axis indicates the distance between
atoms in angstrom units, and the vertical axis indicates the energy,
with lower energy and greater stability represented downward. The
zero point of energy has been chosen to be that of two infinitely
separated, noninteracting atoms.
Two hydrogen atoms infinitely far apart obviously do not interact,
and thus have no bond between them.
the atoms come closer together, little happens until their interatomic
distance decreases to a few angstroms. Then the electron of one
atom begins to be "seen" by the nucleus of the other. Each electron
is attracted by the other nucleus, the electrons become concentrated
between the nuclei, and a bond begins to form. The energy of the
two atoms decreases as the attraction of each nucleus for the other
electron becomes significant.