Oxidation Numbers

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

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

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

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-

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

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