Determine Polyatomic Charge: Sulfate (SO4)

[yyttr2]

In writing a formula from the name of an element I can safely say I know how to figure out the number of oxygen it will have.

But take:

Sulfate: $$So^{-2}_{4}$$
How do you determine the charge of it with just the name?

 

[Char. Limit]

From the name? You don't. However, I have had some success with looking at the charge the central
atom would hold if it were an ion, which is often (but not always; see carbonate and nitrate for counterexamples) the same as the charge of the polyatomic ion, however many oxygens there are. For example, all of the oxyacids of chlorine have the same charge as a chlorine ion, all of the oxyacids of sulfur and phosphorus have the same charge as sulfur and phosphorus ions, and so forth.

This idea that I use only seems to work in periods 3 or greater, and in groups 15 through 17 (or 5A through 7A).

 

[chemisttree]

The suffix -ate is given to the higher of the oxyanions in the series. Without the prefix 'per-', assume the oxygens are all of -2 charge. Assume the the sulfur is in its normal oxidation state of +6 (sulfur has oxidation states of -2, +4 and +6) since you memorized(!) the charges of all of the oxyanions, of course!