Q.What are valence band and conduction band?

Just as interacting atomic orbitals form molecular orbitals, the interacting energy levels of the many closely spaced atoms in a solid form bands -- near-continuous sets of electronic energy levels. Two bands matter most: the valence band, the lower-energy band whose electrons stay tightly bound to their nuclei and cannot move; and the conduction band, the highest-energy band containing electrons, which may be partially filled or empty, and whose electrons (when present) are mobile and delocalised over the whole solid. The energy separation between them is the band gap; a gap too large for thermal energy to bridge is a forbidden zone (insulator), while a small gap lets some valence electrons be thermally promoted into the conduction band (semiconductor). Measured band gaps illustrate the scale: diamond 5.47 eV (insulator), silicon 1.12 eV and germanium 0.67 eV (semiconductors), sodium 0 eV (metal, no gap). In a metal, either the conduction band is already partially filled (e.g. sodium's half-filled 3s band, formed from the bonding-MO/antibonding-MO halves of the interacting 3s orbitals with no gap between them) or a filled band overlaps in energy with an empty one (e.g. magnesium's overlapping filled 3s and vacant 3p bands), so electrons can always move freely and conductivity is high, though it falls with rising temperature as increased lattice vibration disrupts electron flow.