Problem 4-97

Sometimes it is not necessary to calculate the formal charge of all individual atoms

Some groups of atoms in a compound have the same oxidation number as their corresponding ions

"OH" = -1
"NH₄" = +1
"SO₄" = -2
  etc.

Additionally, transition metals in an ionic compound can have different oxidation numbers at the same time, thus imparting a fractional charge to the metal in question.

Fe₃O₄ = (Fe³⁺)(Fe²⁺)(Fe³⁺) + 4 × O^2-

The average ON of Fe is (+8 charge total) /(3 Fe atoms) or +2.666.. average Fe atom.