1. Finding the formula of a 2-element compound e.g. potassium bromide Step 1 Find potassium (K) in the periodic table and identify its group number. Do the same for bromine (Br)

2. Finding the formula of a 2-element compound e.g. potassium bromide Step 2 Use the group number to give the valency of the elements involved For groups 1 to 5 valency = group number For groups 5 to 8 valency = 8 – group number

3. Finding the formula of a 2-element compound e.g. potassium bromide So the valency of potassium is 1 The valency of bromine is 1

4. Finding the formula of a 2-element compound e.g. potassium bromide Step 3 Put the element formulae next to each other, metal first and the valency underneath K Br 1 1

5. Finding the formula of a 2-element compound e.g. potassium bromide Cross over the valencies to give the formula of the compound K Br so: formula is KBr (number ones are not shown) 1

6. Finding the formula of sodium oxide Na in group 1 so valency = 1 (group number) Oxygen in group 6 so valency = 2 (8-gp N o ) Na O 1 2

7. Finding the formula of sodium oxide Formula = Na 2 O

8. Finding the formula of aluminium chloride AlCl 3

9. Finding the formula of calcium oxide Ca gp 2 so valency = 2 O gp 6 so valency =2 Ca 2 O 2 2 of each = 1 of each So formula = CaO

10. Try these Lithium fluoride Magnesium chloride Aluminium oxide Potassium sulphide Carbon chloride Silicon oxide Boron nitride

11. Answers LiF MgCl 2 Al 2 O 3 K 2 S CCl 4 SiO 2 BN

12. What about group 5?? Nitrogen always has a valency of 3 Phosphorus can be 3 or 5 so how do we know which? It is given in the name (using Roman numerals) E.g Phosphorus(V) chloride PCl 5

13. What is the formula of phosphorus (III) chloride PCl 3

14. This is also used with transition metal compounds What is the formula of: iron(II) bromide, cobalt(III) fluoride, titanium(IV) oxide

15. Answers FeBr 2 CoF 3 TiO 2

16. What about compounds with more than 3 elements? Calcium carbonate – limestone Sodium chlorate – an explosive weed killer Sodium hydrogencarbonate – baking powder To work out these formula we have to LEARN the following list of valencies.

17. IonFormulaValency HydroxideOH - 1 NitrateNO 3 - 1 Hydrogen Carbonate HCO 3 - 1 CarbonateCO 3 2 - 2 SulphateSO 4 2 - 2 SulphiteSO 3 2 - 2 PhosphatePO 4 3 - 3 AmmoniumNH 4 + 1 Manganate(VII)MnO 4 - 1 Dichromate(VI)Cr 2 O 7 2 - 2

18. These are then used in exactly the same way as before E.g. Calcium nitrate Nitrate has the formula and valency of NO 3 - Valency of 1 Calcium (gp II) has a valency of 2 Ca(NO 3 ) 2

19. Additional notes Metals-Use Symbols only (also true for Carbon Sulphur Phosphorus) Commmon Gases - Molecules containing 2 atoms (–diatomic) E.g O 2 N 2 Cl 2 H 2 F 2 [Br 2 (l), I 2 (s)]

20. Common Acids HCl Hydrochloric H 2 SO 4 Sulphuric HNO 3 Nitric H 3 PO 4 Phosphoric CH 3 COOH Ethanoic

21. Watch out for ammo-nia/nium NOTE: Ammonia NH 3 Ammonium NH 4 + Valency=1