Elements 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.