Draw the Lewis structures for each of the following molecules or ions. Identify instances where the octet rule is not obeyed; state which atom in each compound does not follow the octet rule; and state how many electrons surround these atoms: (a) PF₆^-, (b) BeCl₂, (c) NH₃, (d) XeF₂O (the Xe is the central atom), (e) SO₄^2- .