# localized electrons in the crystals

Why electrons in low lying levels of individual atoms stay localized in their own atoms in a crystal? Doesn't this contradict Bloch's theorem?

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Yes (that would be the lowest state in the energy band, to which 1s energy level would expand to). But in that case the crystal would be unstable, because the average charge density would be positive then. To make a zero average charge density, you need to take as many electrons as protons in the nuclei. For the simpler model, consider 1 electron per 2 protons - an $\mathrm{H}_2^+$ hydrogen molecular ion. –  firtree May 9 '13 at 10:03