Oxidation: any process by which an entity loses electrons. e.g.2Mg o + O 2 o 2Mg 2+ + O 2- H 2 o + F 2 o 2H + F - Mg and H are oxidised in these examples. Reduction: any process by which an entity gains electrons. Oxygen and fluorine are reduced in the above examples. Nomenclature:positive ions are called “ cations” negative ions are called “ anions” Oxidation state:the sign and magnitude of the charge on an ion (also called oxidation number).

Note: 1. Oxidation state of an element in the uncombined state always equal to zero. 2. The algebraic sum of the oxidation states of all the atoms in the molecular formula of a compound is zero. 3. In complex ions (contain more than one atom) the sum of the oxidation states of all the constituents must equal overall charge on the ion. Common Oxidation States of Elements 1.Hydrogen usually Oxygen usually Halogens (i.e. F, Cl, Br, I) usually Metal ions are invariably positive.

Example: Calculate the oxidation state of S in the S 2 O 3 2- ion. Solution: Oxidation state of oxygen is -2. Sum of oxidation states of all the elements in the ion must equal the charge on the ion.  2 (ox. st. of S) + 3 (-2) = -2 2 (ox. st. of S) - 6 = -2 2 (ox. st. of S) = +4  oxidation state of S = +2

Example: Calculate the oxidation state of S in the S 2 O 8 2- ion. Solution: Oxidation state of oxygen is -2.  2 (ox. st. of S) + 8 (-2) = (ox. st. of S) - 16 = (ox. st. of S) = + 14  oxidation state of S = +7 Example: Calculate the oxidation state of Mn in KMnO 4. Solution: Oxidation state of oxygen is -2.  1 +  ox. st. of Mn + 4 (-2) = 0 1+ ox. st. of Mn - 8 = 0 ox. st. of Mn = + 7  oxidation state of Mn = +7

Nomenclature of Cations Cation Name Al 3+ Aluminium Ba 2+ Barium Na + Sodium Cu + Cuprous or copper (I) Cu 2+ Cupric or copper (II) Fe 2+ Ferrous Fe 3+ Ferric Sn 2+ Stannous Sn 4+ Stannic NH 4 + Ammonium

Nomenclature of Anions AnionName Br - Bromide Cl - Chloride O 2- Oxide S 2- Sulphide OH - Hydroxide NO 3 - Nitrate CO 3 2- Carbonate SO 4 2- Sulphate PO 4 3- Phosphate CN - Cyanide

Oxidation States of Some Common Ions CationsAnions Li + Be 2+ Al 3+ Si 4+ F - O 2- N 3- Na + Mg 2+ Sc 3+ Mn 4+ Cl - S 2- P 3- K + Ca 2+ Y 3+ U 4+ Br - Se 2- Rb + Sr 2+ Ga 3+ Th 4+ I - CO 3 2- Cs + Ba 2+ Sb 3+ Ce 4+ OH - SO 4 2- Cu + Mn 2+ Bi 3+ NO 3 - Ag + Fe 2+ V 3+ NO 2 - Tl + Co 2+ Cr 3+ CN - H + Ni 2+ Fe 3+ MnO 4 - NH 4 + Cu 2+ Co 3+ Zn 2+ Cd 2+

Oxidation and Reduction - Summary l Oxidation = loss of electrons l Reduction = gain of electrons l Oxidation state or number: sign and magnitude of charge on ion l Hydrogen +1; oxygen -2; halides usually -1; metals usually +ve. l Calculation of oxidation states in ions and molecules

Photography light 2 AgBr2Ag + Br 2 light, h