When a large number of atoms are brought together to form a solid, a similar
phenomenon occurs. As the atoms are brought close together, the various
isolated-atom energy levels begin to split. This splitting is shown in Figure
12.16b for six atoms in proximity. In this case there are six energy levels corresponding to six different linear combinations of isolated-atom wavefunctions.
The width of an energy band (designated E in Fig. 12.16) depends only on the
number of atoms close enough to interact strongly, which is always a small number. If we consider the total number of atoms in a solid (N 1023 atoms/cm3
),
we find a very large number of levels (determined by N) spaced within the
width E, so the levels may be regarded as a continuous band of energy levels
(see Fig. 12.16c). In the case of sodium, it is common to refer to the continuous
distribution of allowed energy levels as the 3s band, because it originates from
the 3s levels of individual sodium atoms. In general, a crystalline solid has numerous allowed energy bands, one band arising from each atomic energy level.
Figure 12.18 shows the allowed energy bands of sodium. Note that energy gaps,
or forbidden energy regions, separate the allowed energy bands. Forbidden energy regions arise from the separation between different atomic levels and will
always be present unless individual atomic levels broaden so much that they
overlap, as do the 3s and 3p bands in sodium.
If the solid contains N atoms, each energy band has N energy levels. The 1s,
2s, and 2p bands of sodium are each full of electrons, as indicated by the dark
gray-shaded areas in Figure 12.18. A level whose orbital angular momentum is
can hold 2(2 1) electrons. The factor of 2 arises from the two possible
electron-spin orientations, while the factor 2 1 corresponds to the number
of possible orientations of the orbital angular momentum. The capacity of
each band for a system of N atoms is 2(2 1)N electrons. Hence the 1s and
2s bands each contain 2N electrons ( 0), while the 2p band contains 6N
electrons ( 1). Because sodium has only one 3s electron and there is a total
of N atoms in the solid, the 3s band contains only N electrons and is only half
full. The 3p band, which is above the 3s band, is completely empty.