STANDARD GRADE CHEMISTRY Writing Formulae. Formulae of compounds can be written using either 1. The name of the compound. “mono” meansone “di”meanstwo “tri”meansthree “tetra” means four “penta” means five If any of these prefixes appears in the name of the compound we use this to write the formula. e.g.carbon monoxideisCO carbon dioxideis CO 2

Standard Grade Chemistry 2.Using Valency Rules If the name does not give any clues about the formula, we use valency rules. e.g.sodium oxide write symbolsNaO write valency12 cross over valency21 cancel if necessary write formulaNa 2 O Note:- For transition metals the valency is usually written as a Roman numeral in brackets after the name of the transition metal e.g. In iron(III) oxide, the iron has a valency of 3.

 3. Formulae involving Complex (group) Ions If the name of the compound ends in “-ate” or “ –ite”, it contains a complex ion (watch also for hydroxide). Complex ions are found in your data book (p4). e.g. Copper(II) nitrate write symbols or formula CuNO 3 - write valency21 (for complex ions, this is the charge) cross over valency12 Use brackets and cancel if necessary write formulaCu(NO 3 ) 2

 4. Formulae Showing Charges Sometimes you are asked to show the formula with charges (ionic formula) e.g. sodium carbonate write symbols or formula NaCO 3 2- write valency12 (for complex ions, this is the charge) cross over valency21 Use brackets and cancel if necessary write formula leaving charges(Na + ) 2 CO 3 2- Write ionic formulae for the compounds listed above.

Calculations for you to try. Standard Grade Chemistry SO 3 1. Write the formula for:- (a)Sulphur trioxide (b)Dinitrogen monoxide (c)Phosphorus pentachloride (d) Carbon terachloride. N2ON2O PCl 5 CCl 4 2. Write the formula for:- (a)Sodium oxide. (b)Aluminium iodide (c)Iron(III) oxide (d) Silver(I) oxide. AlI 3 Na 2 O Fe 2 O 3 Ag 2 O 3. Write the formula for:- (a)Sodium carbonate (b)Aluminium nitrate (c)Iron(III) hydroxide (d) Nickel(II) sulphate Na 2 CO 3 Al(NO 3 ) 3 Fe(OH) 3 NiSO 4

4. Write the ionic formula for:- (a)Lithium hydrogencarbonate (b)Aluminium hydroxide (c)Iron(II) sulphate (d) Copper(II) nitrate. Li + HCO 3 - Al 3+ (OH - ) 3 Fe 2+ SO 4 2- Cu 2+( NO 3 - ) 2