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 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 No)

Na O

1 2

Finding the formula of sodium oxide

Formula = Na2O

Finding the formula of aluminium chloride

AlCl3

Finding the formula of calcium oxide

Ca gp 2 so valency = 2 O gp 6 so valency =2 Ca2O2

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 MgCl2 Al2O3

K2S

CCl4 SiO2

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 PCl5

What is the formula of phosphorus (III) chloride

PCl3

This is also used with transition metal compounds

What is the formula of: iron(II) bromide, cobalt(III) fluoride, titanium(IV) oxide

Answers

FeBr2

CoF3

TiO2

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.

Ion Formula Valency

Hydroxide OH- 1

Nitrate NO3- 1

Hydrogen Carbonate

HCO3- 1

Carbonate CO32- 2

Sulphate SO42- 2

Sulphite SO32- 2

Phosphate PO43- 3

Ammonium NH4+ 1

Manganate(VII) MnO4- 1

Dichromate(VI) Cr2O72- 2

These are then used in exactly the same way as before

E.g. Calcium nitrate Nitrate has the formula and valency of NO3

-

Valency of 1 Calcium (gp II) has a valency of 2 Ca(NO3)2

Additional notes

Metals-Use Symbols only (also true for Carbon Sulphur Phosphorus)

Commmon Gases - Molecules containing 2 atoms (–diatomic)

E.g O2 N2 Cl2 H2 F2 [Br2(l), I2(s)]

Common Acids HCl Hydrochloric H2SO4

Sulphuric

HNO3

Nitric H3PO4 Phosphoric CH3COOH Ethanoic

Watch out for ammo-nia/nium

NOTE: Ammonia NH3

Ammonium NH4+ Valency=1