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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