In the formula for any substance the sum of the oxidations numbers of all the atoms in the formula is equal to the charge shown. Thus: For elements, such as Ar, O2, S8, etc. in the uncombined state the oxidation number for each atom must be 0, since no charge is shown and the atoms are equal to each other. For monoatomic ions the oxidation number equals the charge. For a compound the sum of the oxidation numbers of the atoms equals 0. For a polyatomic ion the sum of the oxidations numbers of the atoms equals the charge on the ion. In compounds fluorine is always assigned an oxidation number of -1. In compounds oxygen is usually assigned an oxidation number of -2. Exception 1: in peroxides it is -1 while in superoxides it is -1/2. These will generally be obvious due to other rules (or the names). Exception 2: in combination with fluorine oxygen can be positive due to Rule 2 above, thus for OF2 oxygen is assigned an oxidation number of +2. In compounds hydrogen is usually assigned an oxidation number of +1 Exception: in metallic hydrides hydrogen is assigned an oxidation number of -1. These exceptions will be fairly obvious: NaH, CaH2, etc. Alkali metals in compounds will always (for our class) be assigned an oxidation number of +1. Alkaline-earth metals in compounds will always (for our class) be assigned an oxidation number of +2 Aluminum will always (for our class) be assigned an oxidation number of +3, other elements in this Group will usually be assigned an oxidation number of +3.

### MgH2

Mg

Rule 6 assigns a value of +2

H

Rule 1 states that the ON of the compound = its charge = 0 for MgH2

So ON for 2(H) + ON for Mg = 0

2(H) + (+2) = 0; 2(H) = 0 - (+2)

ON for H = -1

### HClO4

Rule 1 & 3 & 4 gives +1 + Cl + 4(-2) = 0

Cl = -1 - (-8) = +7

### NH4+

Rule 1 & 4 gives 4(+1) + N = +1,

N = +1 - (+4) = -3

### NO2-

Rule 1 & 3 gives N + 2(-2) = -1,

N = -1 - (-4) = +3