WJEC Chemistry for AS Level Student Book: 2nd Edition

The formula for ionic compounds can be calculated by following these steps: 1. Write the symbols of the ions in the compound. 2. Balance the ions so that the total of the positive ions and negative ions adds to zero. (The compound itself must be neutral.) 3. Write the formula without the charges and put the number of ions of each element as a small number following and below the element symbol. Worked examples Example 1 Magnesium oxide 1. The ions are Mg 2+ and O 2− . 2. To make the total charge zero, we need one Mg 2+ ion for every O 2− ion (+2 −2 = 0). 3. Formula is MgO (the ‘1’ does not need to be included). Example 2 Sodium sul de 1. The ions are Na + and S 2− . 2. To make the total charge zero, we need two Na + ions for every S 2− (+1 +1 −2 = 0) i.e. Na + Na + S 2− . 3. Formula is Na 2 S. Example 3 Calcium nitrate 1. The ions are Ca 2+ and NO 3 − . 2. Two NO 3 − ions are needed to balance the charge on one Ca 2+ ion (−1 −1 +2 = 0) i.e. Ca 2+ NO 3 − NO 3 − . 3. Formula is Ca(NO 3 ) 2 (note the use of a bracket around the NO 3 before adding the 2). Oxidation numbers As you have seen on the previous pages, there are differences in the ratios of the atoms that combine together to form compounds, e.g. H 2 O and HCl. 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. For example, the iron(II) ion, Fe 2+ , needs the addition of two electrons to make a neutral atom, therefore it 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 use of oxidation numbers can be extended to covalent compounds. Some elements are assigned positive oxidation numbers and others are assigned negative oxidation Study point It is useful to use simple steps in determining oxidation numbers. E.g. What is the oxidation number of nitrogen in the NO 3 − ion? Step 1 The oxidation number of each oxygen is −2. Step 2 The total for O 3 is −6. Step 3 The overall charge on the ion is −1. Step 4 The oxidation number of nitrogen is (−1) − (−6) = +5. Key term Oxidation number is the number of electrons that need to be added to (or taken away from) an element to make it neutral. 4 Knowledge check Give the formula for: (a) Aluminium oxide (b) Potassium carbonate (c) Ammonium sulfate. Study point Sometimes the term oxidation state is used instead of oxidation number. The only difference is that we say for Fe 2+ , for example, the oxidation number of Fe is +2 but the oxidation state of this ion is written as Fe(II). 12 WJEC Chemistry for AS Level