When two atoms bond together, two molecular orbitals are formed.
When three atoms bond together, three molecular orbitals are formed.
In the extended solid, many atoms interact with one another, and there will be the same number of molecular orbitals, delocalized over the entire solid, as the number of atomic orbitals being combined.
The orbitals are so tightly packed together in energy that they comprise a so-called "band."
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