rules for assigning oxidation states

Instead of redrawing every bond and arguing about which atom is greedier, chemists agreed on a short checklist that gets you the oxidation states in seconds. Think of it as a priority list, like the rules of a card game: you apply the surest assignments first, and let the leftover atom soak up whatever is needed to make the totals come out right.

The usual ranking goes like this. A free element (O2, Na metal, S8) is 0. A monatomic ion equals its charge (Na+ is +1, Cl- is -1). The sum of all oxidation states equals the overall charge of the species. Then come the reliable defaults: fluorine is always -1; group 1 metals are +1 and group 2 are +2; hydrogen is usually +1 (but -1 in metal hydrides like NaH, where it is the more electronegative partner); oxygen is usually -2 (but -1 in peroxides such as H2O2, -1/2 in superoxides, and even +2 in OF2, where the rarer, more electronegative fluorine wins). You assign these in order, then solve for any remaining atom using the sum rule.

These rules let you read redox chemistry off a formula without drawing a single structure, but they are conventions, not laws of nature — and the exceptions (peroxides, hydrides, OF2) are exactly the cases that catch beginners. They can also hand you fractional or 'average' values, as in Fe3O4, where iron averages +8/3 because the solid really contains a mix of Fe2+ and Fe3+. The fraction is honest about your ignorance of the individual sites; it does not mean an atom literally carries two-thirds of a charge.