How can you use formal charge to determine a) when an atom should be electron deficient or b) when an atom should have an expanded octet in a Lewis Structure.

Using the Formula Fc = (Valence Electrons) - (Unbound Electrons) - (N bonds in the atom)

Explanation:

Concept: Formal Charge

Let's remember the concept of formal charge as the electric charge of an atom in a given molecule, it is calculated as it follows:

Fc = (Valence Electrons) - (Unbound Electrons) - (N bonds in the atom)

So to answer the question and by knowing the molecule, we can tell

a) Fc + Unbound Electrons + N Bonds of the atom = Valence Electrons, if the valence electrons are fewer than the actual electrons of the atom, it means it is missing electrons.

b) let' s remember the concept of expanded octet as an atom and the Lewis Structure having more than 8 electrons.

So if you sum the Valence Electrons and the N bonds in the atom multiplied by two (There are two electrons in 1 bond) and this number is greater than 8, you could say it has an expanded octet