Unit 11: Redox Oxidation numbers

What is an Oxidation number? Oxidation numbers are assigned to an atom in a molecule based on the distribution of electrons.

Rules for assigning Oxidation Numbers An atom in its elemental state has an oxidation number of 0.  Ex) Mg(s) 0 Al(s) 0, H 2 (g) 0, Br 2 (g) 0 The oxidation number of a monatomic ion is equal to the charge of the ion.  Ex) K +, Mg 2+, Br -, S 2- In compounds, atoms of Group 1, 2, and aluminum have oxidation numbers of +1,+2,+3 respectively.  Ex) NaCl  Na +1 Cl -1,  MgO  Mg +2 O -2,  AlCl 3  Al +3 Cl 3 -1

Rules for assigning Oxidation Numbers The oxidation number of hydrogen in a compound is +1, unless it is combined with a metal atom, then it is -1.  Ex) HCl  H +1 Cl -1, CaH 2  Ca +2 H 2 -1 The oxidation number of fluorine is always -1  Ex) Ca +2 F 2 -1, Na +1 F -1 The oxidation number of oxygen is -2, except with compounds of fluorine where it is +2, and peroxides such as H 2 O 2 it is -1.  Ex) Mg +2 O -2  OF 2  O +2 F 2 -1

Rules for Assigning Oxidation Numbers The sum of all oxidation numbers in a compound is 0.  Ex) H 2 O  H 2 +1 O -2, (2)(+1) + (-2) = 0  NaCl  Na +1 Cl -1 (+1) + (-1) = 0 The sum of all oxidation numbers in a polyatomic ion equals the charge of that ion.  Ex) NO 3 -  (N +5 O 3 -2 ) - (+5) + 3(-2) = -1  OH -  (O -2 H +1 ) - (-2) + (+1) = -1

Practice… a)N 2 b) NCl 3 c) SF 6 d) MgBr 2 e) SO 4 2- f) Mg 2+ Answers: a)N 2 0 b) N +3 Cl 3 -1 c)S +6 F 6 -1 d) Mg +2 Br 2 -1 e) (S +6 O 4 -2 ) 2- f) Mg 2+ Assign oxidation numbers to the elements in each of the following: