1. Oxidation Numbers

2. Oxidation number Number of electrons lost or gained by an atom
A lot of the time it is the charge of the ion
What is the oxidation number for the following atoms as ions? Na O Cl Mg

3. Oxidation and Reduction
Oxidation: loss of electrons from atoms of a substance
Example: Na Na+ + e-
Reduction: gain of electrons by atoms of a substance
Example: Cl2 + 2e-  2 Cl-
OIL RIG: Oxidation is Loss of electrons, Reduction is Gain of electrons

4. Examples
Is each of the following an example of oxidation or reduction?
Ca  Ca2+ + 2e-
N + 3e- N3-

5. Rules for Determining Oxidation Numbers
1. The oxidation of an atom of an element is zero.
Example: C, O2, S8
2. The oxidation of a monoatomic ion is equal to the charge on the ion
Example: Ca2+, N3-

6. 3. The oxidation number of the more electronegative atom in a molecule or complex ion is the same as the charge it would have if it were an ion
Example: NH3, SiCl4
4. The most electronegative element, F, always gets an oxidation number of -1.
Example: HF, CF4

7. 5. The oxidation number of oxygen in compounds is always -2, except in peroxides like H2O2 where it is -1, and bonded to fluorine where it is +2.
Example: H2O2, OF2 , H2O
6. The oxidation number of hydrogen in most compounds is +1 when bonded to nonmetal and -1 when bonded to metal
Example: LiH, H2O, NaOH, AlH3

8. 8. The sum of the oxidation numbers in a neutral compound is zero.
7. Metals in groups 1 & 2 and aluminum in group 13 form compounds in which the metal atom always has a positive oxidation number equal to the number of its valence electrons.
Example: NaCl, MgCl2, AlCl3
8. The sum of the oxidation numbers in a neutral compound is zero.
Example: NaCl, CaBr2

9. 9. The sum of the oxidation numbers of the atoms in a polyatomic ion is equal to the charge of the ion.
Example: NH4+