THE MODERN PERIODIC TABLE
The close similarity of the atomic spectra of other atoms to that of
hydrogen indicates that, as we progressively increase the number of
protons in the nucleus and the extranuclear electrons in the atom for
a series of elements of increasing atomic number, the additional electrons
enter orbitals of the type originally suggested by wavemechanics
for hydrogen. The orbitals are filled in order of ascending
energy and when several equivalent energy levels are available, each
is occupied by a single electron before any pairing of electrons with
opposed spin occurs.
The order of increasing energy for the orbitals can be deduced from
the modern periodic table although for elements of high atomic number
(when the electron energy levels are close together) the precise
positioning of an electron may be rather uncertain. The filling of the
energy levels for the first ten elements, hydrogen to neon, atomic
numbers 1-10 is shown in Table 12.