The oxidation state is an indication of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic.
The guidelines for assigning oxidation states (numbers) are given below:
- The oxidation state of any element such as Fe, H2, O2, P4, S8 is zero (0).
- The oxidation state of oxygen in its compounds is -2, except for peroxides like H2O2, and Na2O2, in which the oxidation state for O is -1.
- The oxidation state of hydrogen is +1 in its compounds, except for metal hydrides, such as NaH, LiH, etc., in which the oxidation state for H is -1.
- The oxidation states of other elements are then assigned to make the algebraic sum of the oxidation states equal to the net charge on the molecule or ion.
- The following elements usually have the same oxidation states in their compounds:
- +1 for alkali metals - Li, Na, K, Rb, Cs;
- +2 for alkaline earth metals - Be, Mg, Ca, Sr, Ba;
- -1 for halogens except when they form compounds with oxygen or one another;