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)
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
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
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
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
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
Finding the formula of sodium oxide Formula = Na 2 O
Finding the formula of aluminium chloride AlCl 3
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
Try these Lithium fluoride Magnesium chloride Aluminium oxide Potassium sulphide Carbon chloride Silicon oxide Boron nitride
Answers LiF MgCl 2 Al 2 O 3 K 2 S CCl 4 SiO 2 BN
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
What is the formula of phosphorus (III) chloride PCl 3
This is also used with transition metal compounds What is the formula of: iron(II) bromide, cobalt(III) fluoride, titanium(IV) oxide
Answers FeBr 2 CoF 3 TiO 2
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.
IonFormulaValency HydroxideOH - 1 NitrateNO Hydrogen Carbonate HCO CarbonateCO SulphateSO SulphiteSO PhosphatePO AmmoniumNH Manganate(VII)MnO Dichromate(VI)Cr 2 O
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
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)]
Common Acids HCl Hydrochloric H 2 SO 4 Sulphuric HNO 3 Nitric H 3 PO 4 Phosphoric CH 3 COOH Ethanoic
Watch out for ammo-nia/nium NOTE: Ammonia NH 3 Ammonium NH 4 + Valency=1