The model just described for Li is oversimplified; bands
are also formed by the overlap of higher energy (unoccupied)
atomic orbitals, and the 2p band actually overlaps with the 2s
band to some extent since the s–p separation in atomic Li is
relatively small. This is also true for Be and, of course, this
is of great significance since the ground state electronic
configuration of Be is [He]2s2; were the energy separation
of the 2s and 2p bands in Be large, the 2s band would be
fully occupied and Be would be an insulator. In reality, the
2s and 2p bands overlap, and generate, in effect, a single,
partially occupied band, thereby giving Be its metallic
character. Figure 5.12a–c illustrates that: