If two identical atoms are very far apart, they do not interact, and their electronic energy levels can be considered to be those of isolated atoms. Suppose
the two atoms are sodium, each having an outermost 3s electron with a specific
energy. As the two sodium atoms are brought closer together, their wavefunctions overlap, and the two degenerate, isolated 3s energy levels are split into
two different levels, as shown in Figure 12.16a. We can understand this splitting
by considering the appropriate electronic wavefunctions for the case of widely
separated atoms and the case of neighboring atoms. (For a more complete and
careful treatment of this splitting, see the discussion of covalent bonding in
Section 11.4.) Figure 12.17 shows idealized isolated atom wavefunctions 1 and
2 as well as the linear combinations 1 2 and 1 2 that represent
approximate electronic wavefunctions for the two atoms close together. Note
that an electron in the state 1 2 has a substantial probability of being
found midway between the ion cores, while in the state 1 2 the probability
density vanishes at the midpoint. Since the electron spends part of its time midway between the two attractive ion cores in the state 1 2, the electron is
more tightly bound (has lower energy) in the state 1 2 than in 1 2.
This leads to the two different 3s energy levels shown in Figure 12.16a.5