MOLECULES AND SOLIDS!

Molecules show “bands” instead
of energy levels. This is because there are so many more quantum
degrees of freedom that a molecule displays, the quantum
equivalent of classical rotations and vibrations, plus the many
ways electron state functions in neighboring atoms can overlap.

In solids, the number of
different ways electron state functions in neighboring atoms can
overlap becomes almost limitless, so the energy levels for
electrons merge into broad bands that are practically continuous
since they contain billions and billions of closely spaced
different energy levels!

The basis of all modern
electronics... “doped” semiconductors forming a diode... a
switch with no moving parts, as small as you please.