by the overlap of two atomic orbitals from different atoms
Because that's the way physics works. There really isn't a "why". It's kind of like asking why there are negative numbers. There just are.
iono
Molecular consists of multiple atomic orbitals
Molecular compounds certainly can be formed from semimetals, but not all the compounds of semimetals are molecular.
"just as an atomic orbital belongs to a particular atom, a molecular orbital belongs to a molecule as a whole" -prentice hall chemistry
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