in the same vertical column in the periodic table have similar chemical
properties and, for the representative elements at least, the same
outer electronic configuration. This systematic arrangement of elements,
as shown at the top of page 12, is the
periodic table, based on chemical experience rather than theory,
that any serious atomic theory must explain satisfactorily.
The wave mechanics discussed in the previous section provides this
explanation. The energy-level diagram that we saw on page
7, and which is repeated opposite, describes the different energy
states that are available to an atom. As the atomic number and the
positive charge on the nucleus increase, all of these levels are
lowered in energy because the nucleus pulls on the electrons more
tightly. In spite of this, the relative order of levels remains
much the same. With minor differences, this sequence of levels holds
for all atoms.
We can think of building up an atom by first placing the correct
positive charge on the nucleus, and then adding electrons around
the nucleus, one at a time, until enough electrons have been added
to counterbalance the positive charge. In the ground state, or lowest-energy
state of the atom, each additional electron will go into the lowest
energy level still available. In this way the electronic structure
of any atom is determined by the successive filling of energy levels
from the lowest upward. We have already used this process to show
the buildup of the first ten elements.