Summary of most important ions to know How to work out the formula from the ionic charge What is the formula of Lithium Sulphate? 1. Identify the ionic charges of the two ions Lithium is in group 1 so has a +1 charge, Li+ sulphates have a -2 charge, SO4 2- 2. Combine the ions together to get a neutral compound. i.e. combine so that the total +ve charge cancels out the total –ve charge Li + SO4 Li+ 2- We need two lithium ions to cancel out the -2 charge on the sulphate The formula is therefore Li2SO4 What is the formula of Calcium phosphate? 1. Identify the ionic charges of the two ions Calcium is in group 2 so has a +2 charge, Ca2+ Phosphates have a -3 charge, PO4 3- 2. Combine the ions together to get a neutral compound. i.e. combine so that the total +ve charge cancels out the total –ve charge We need to multiply up to get the same charge. Three calcium ions would produce +6 charge to cancel out the -6 charge on two phosphate ions The formula is therefore Ca3 (PO4 )2 Only use brackets when there is more than one of the compound ion in the formula. e.g. Copper nitrate is Cu(NO3 )2 Calcium hydroxide is Ca(OH)2 Ammonium sulphate is (NH4 )2SO4 2. +1 +2 +3 -3 -2 -1 Group 1 Hydrogen Silver Gold Ammonium (NH4+ ) Group 2 Zinc Copper (II) Iron (II) Tin Lead Group 3 Iron (III) Group 5 Phosphate (PO4 3-) Group 6 Carbonate (CO3 2-) Sulphate (SO4 2-) Group 7 Nitrate (NO3 -) Hydroxide (OH-)
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Formulae and equations (a) the writing of formulae of ionic compounds from ionic charges, including: (i) prediction of ionic charge from the position of an element in the periodic table (ii) recall of the names and formulae for the following ions: NO3 –, CO3 2–, SO4 2–, OH–, NH4 +, Zn2+ and Ag+ Note that ‘nitrate’ and ‘sulfate’ should be assumed to be NO3 – and SO4 2–. Charges on ions other than in (i) and (ii) will be provided.