energy band
/ EN-ur-jee band /
In a single atom an electron can only sit at certain fixed energy steps, like rungs on a ladder — never in between. Now imagine bringing trillions of identical atoms close together to make a crystal. Each lonely rung splits into a dense forest of slightly different rungs, one for every atom, packed so tightly that they smear into what looks like a continuous ramp. That smeared-out ramp of closely spaced energy steps is an energy band.
More precisely, an energy band is a continuous range of energies that electrons in a crystal are allowed to have. It appears because the outer electrons of neighbouring atoms overlap and share, so a single atomic energy level turns into a band containing as many closely spaced levels as there are atoms. Between one band and the next there can be a forbidden range with no allowed energies at all. Each band can hold only a limited number of electrons, so the crystal fills its bands from the bottom up.
Energy bands matter because whether a material is a metal, a semiconductor, or an insulator comes down almost entirely to how its bands are arranged and how full they are. A common misconception is to picture a band as one single energy; it is really an enormous number of distinct allowed energies crowded into a range, and the spacing only looks continuous because the levels are so absurdly close together.
Take two hydrogen atoms and push them together to make a molecule: the single 1s level of each atom splits into two — one slightly lower, one slightly higher. Now do this with a billion atoms in a line, and those two levels become a billion levels packed into a narrow range. That dense cluster is an energy band, born from nothing more than atoms sharing their electrons.
Many atoms sharing electrons turn one sharp atomic level into a smear of levels — a band.
Bands are not always full of electrons — a band is the set of allowed seats, while how many of those seats are actually occupied is a separate matter that decides the material's electrical behaviour.