If I had to write down the oxidation states of iron (II) and sulphate ions, should I write them as +2 and -2 or 2+ and 2-?

1 Answer
Aug 4, 2016

For iron (II) sulfate:

Oxidation state of iron =+2
Oxidation state of sulfur =+6
Oxidation state of oxygen =-2


Two things to remember:

1) Oxidation states are written with the sign before the number, like +2 for iron. Total charge in a chemical formula is a different concept, that's what has the number first like #"SO"_4^{color(blue)(2-)}#.

2) We usually use oxidation states for individual elements or atoms, not the entire group. Thus for sulfate we need to consider the individual sulfur and oxygen atoms.

Then for the sulfate ion:

  • Oxygen is generally -2 unless bonded to other oxygen atoms or fluorine atoms, and sulfate ion does not have that exception.

  • The sum of of oxidation states of the individual atoms in a group must match the total charge. Sulfate ion has two negative charges and we just saw that each of the four oxygen atoms has oxidation state -2. To make the numbers add up we need to have the sulfur with oxidation +6.

Of course the iron ion, with only one atom, has an oxidation state matching its total charge of +2.