WJEC Chemistry for AS: Study and Rev Guide

A method of expressing the combining power of elements is oxidation number. The oxidation number of an element is the number of electrons that need to be added to (or taken away from) an element to make it neutral. E.g., the ion magnesium, Mg 2+ , forms when magnesium loses two electrons, therefore it needs the addition of two electrons to make a neutral atom and has the oxidation number +2. The chloride ion, Cl – , needs to lose an electron to make a neutral atom; therefore it has the oxidation number –1. The following table gives the rules for assigning oxidation numbers: Rule Example The oxidation number of an uncombined element is zero. Metallic copper, Cu: oxidation number 0 Oxygen gas, O 2 : oxidation number 0. The sum of the oxidation numbers in a compound is zero. In an ion, the sum equals the overall charge. In CO 2 the sum of the oxidation numbers of carbon and oxygen is 0. In NO 3 – the sum of the oxidation numbers of nitrogen and oxygen is –1. In compounds, the oxidation numbers of Group 1 metals is +1 and Group 2 metals is +2. In MgBr 2 the oxidation number of magnesium is +2 (oxidation number of each bromine is –1). The oxidation number of oxygen is –2 in compounds except with fluorine or in peroxides (and superoxides). In SO 2 the oxidation number of each oxygen is –2 (oxidation number of sulfur is +4). In H 2 O 2 the oxidation number of oxygen is –1 (oxidation number of hydrogen is +1). The oxidation number of hydrogen is +1 in compounds except in metal hydrides. In HCl the oxidation number of hydrogen is +1 (oxidation number of chlorine is –1). In NaH the oxidation number of hydrogen is –1 (oxidation number of sodium is +1). In chemical species with atoms of more than one element, the most electronegative element is given the negative oxidation number. In CCl 4 , chlorine is more electronegative than carbon, so oxidation number of each chlorine is –1 (oxidation number of carbon is +4). Example What is the oxidation number of manganese in MnO 4 – ? The oxidation number of each oxygen is –2, so the total for oxygen is –2 × 4 = –8 The total charge on the ion is –1 = ox. no. Mn + total ox. no. O Therefore –1 = ox. no. Mn + (–8) Ox. no. Mn = +7 Oxidation numbers Pointer You do not need to know the definition of oxidation number. Grade boost Learn the rules for assigning oxidation numbers. QUICKFIRE  What is the oxidation number of: a) Carbon in CH 4 b) Phosphorus in Na 3 PO 4 c) Nitrogen in KNO 2 ? 9 1.1 Formulae and equations