Charge of the Central Atom [ENDORSED]

[Achang1L]

I was rewatching Monday's lecture about finding formal charge and I noticed that in the section where Lavelle goes over when not to do too many double bonds, he explains because of the fact that certain atoms have greater electron affinities than others so it makes sense to stop after a certain number of double bonds. Does this mean that the central atom's charge should always be equal to zero or positive, such as sulfur inside of the SO4- molecule? Or can the central atom also be negative? Also, can somebody please explain why Fl or any of the elements in the same group can't have a positive charge?

[Chem_Mod]

You want to avoid putting a negative formal charge on a central atom because it usually has the lowest ionization energy and tends to share electrons to its outer bonding atoms but exceptions are possible. You must never have a positive formal charge on an electronegative atom because electronegativity is the propensity for electrons to be attracted to one of the two atoms in the bonding pair. Thus, the electronegative atom will be electron-rich, not deficient, and never have a positive formal charge.