IONIC

COVALENT

OXIDATION NUMBER A positive or a negative number assigned to a combined atom according to a set of arbitrary (agreed upon) rules.

RULES IN ASSIGNING OXIDATION NUMBER

1. The oxidation number of an atom of a free element is zero 1. The oxidation number of an atom of a free element is zero. This is true even for polyatomic (many atoms) elements. Ex. Na Al O2 S8 0 0 0 0

2. The oxidation number of a monoatomic (one) ion is equal to its charge. Ex . Na +1 Mg +2 Cl - +1 +2 -1

3. The oxidation number of oxygen in a compound is usually -2 3. The oxidation number of oxygen in a compound is usually -2. However, in peroxides (O2 in compound), oxygen has a lower oxidation state, -1. When bound to fluorine, the oxidation number of oxygen is +2. (Ex) H2 O H2 O2 O F2 +1 -2 +1 -1 +2 -1

4. The oxidation number of hydrogen in a compound is usually +1 when combined with a non-metal. In the presence of a metal, its oxidation number is -1. (Ex) H Cl N H3 Li H Mg H2 +1 -1 -3 +1 +1 -1 +2 -1

5. In combination of non-metals, the oxidation number of the less electronegative elements is positive and the more electronegative element, negative. (Ex) H2O H F P2 O5 +1 -2 +1 -1 +5 -2

6. The sum of all oxidation numbers of all the atoms in a neutral compound is zero. (Ex) Na Cl K2 S +1 -1 +1 -2 1(+1) +1 (-1) = 0 2(+1) + 1(-2) = 0

7. The sum of the oxidation numbers of all atoms in the formula of a polyatomic (many atoms) ion is equal to its charge. (Ex) N O3 - O H- +5 -2 -2 +1 1(+5) + 3(-2) = -1 1(-2) + 1(+1) = -1