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The expanded octet comes from access to d orbitals, which can be used to create additional bonds to the 4 allowed by the s and p orbitals. In general, if an atom in a molecule can achieve a lower formal charge and make the molecule have a more logical formal charge by making more than 4 bonds (therefore violating the octet rule), it will.
In general, H, He, and Li want 2 electrons because that satisfies the 1s orbital. Elements in period 3 and any period after can accommodate more than 8 valence electrons because of the d orbitals. Always check formal charge when drawing Lewis structures to confirm that the structure you have drawn is the most stable; this is the best way to prove whether or not the octet rule should apply or not.
When a central atom has empty d-orbitals close in energy to valence orbitals, it can accommodate an expanded octet. Basically, atoms in the p-block of period 3+ can accommodate an expanded octet, but the size of the central atom also matters. It has to be physically capable of forming bonds to more extra atoms.
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