Magnesium has two valence electrons, and the band formed from 3s orbitals is exactly filled by two electrons per atom. Which correctly explains why magnesium is nevertheless a metal?

Answer: C. The 3s band overlaps the empty 3p band, so there are continuous empty levels just above the Fermi level

In a solid, the atomic 3s and 3p orbitals each broaden into bands, and in magnesium their energy ranges overlap. Electrons therefore enter the 3p band before the 3s band is completely filled, and with empty levels continuing without a break just above the Fermi level, even a small electric field accelerates electrons and current flows. This is a metal by band overlap. If there were a gap of about 1 eV, magnesium would behave as a semiconductor and its conductivity would fall at low temperatures. The idea that a valence electron is tightly bound cannot explain metallic conduction either. Be and Ca, also in group 2, are metals for the same reason.