Formula for polyatomic and radical ions

[Wai]

Hi everyone,

 

I'm just curious, is there a rule or pattern for calculating the charge of a polyatomic or radical ion such as sulfate and cyanide? For example, carbonate has a charge of 2-, and phosphite has a charge of 3-. How can you work out their charges without "memorising" them off a sheet?

 

Thanks

[woelen]

Look at the place of the central atom in the periodic table.

 

Halogens are at the last row (-1) before the inert gases, sulphur, selenium and tellurium are two rows (-2) before the inert gases, and phosphorus and arsenic are three rows (-3) before the inert gases.

 

Exceptions are nitrate and nitrite, carbonate. These are -1, -2 instead of -3, -4.

 

Also, keep in mind that phosphate has charge -3, but phosphite has charge -2 (you say -3). But this is not an exeption to the 'rule' I gave above. Phosphite only can give up two hydrogens, so the acid H3PO3 is diprotic and not triprotic. Hypophosporous acid is H3PO2, and that only can give up 1 hydrogen ion, so hypophosphite ion is H2PO2(-).

[Wai]

Whoops... typo there. Thanks