1. Formula for Compounds (ending in –ide)
Compounds who’s names end in –ide contain only 2 elements.
Their chemical formula can easily be worked out by considering the valency of each element.

2. Chemical Formulae
The valency (combining power) of an element, is the number of single covalent bonds it can form.
The valency can be deduced from the group number:
Group Number 1 2 3 4 5 6 7
Valency

3. Writing Formula - Steps
Write the symbols
Underneath, write the valency
Swap them over and write numbers as a subscript.
Cancel out any common factor
Leave out ‘1’ if present.

4. Worked Example 2: Work out the formula for calcium chloride
Ca Cl
2 1
Ca1 Cl2
CaCl2

5. Further Examples: Pg28 Q9 Boron fluoride Carbon sulphide
Hydrogen fluoride
Sodium sulphide
Aluminium oxide
6. Magnesium phosphide
7. Hydrogen sulphide
8. Potassium oxide
9. Carbon chloride
10. Calcium hydride
BF3
CS2 HF
Na2S
Al2O3
6. Mg3P2
7. H2S
8. K2O
9. CCl4
10. CaH2

6. Lesson Starter Write the formula for the following:
Lithium sulphide
Calcium nitride
nitrogen oxide
Magnesium hydride
Sodium iodide
phosphorous chloride
Aluminium sulphide
Carbon hydride
Magnesium oxide
Hydrogen oxide
Li2S
Ca3N2
N2O3
MgH2
NaI
PCl3
Al2S3
CH4
MgO
H2O

7. Ionic Substances
In an ionic substance the valency is given by the charge on the ion.
Transition metals can form a number of different ions, and so their valency is given by roman numerals.
I = 1
II = 2
III = 3
IV = 4
V = 5
VI = 6
VII = 7

8. Worked example: Work out the formula for iron(III)oxide Fe O
Fe2 O Fe2O3

9. Further Examples: Iron(II) chloride Manganese(IV) oxide
Lead(II) iodide
Silver(II) fluoride
Copper(I) oxide
FeCl MnO2 3. PbI2
4. AgF Cu2O

10. Group Ions
Ions containing 2 or more different types of atom are known as group ions.
The formula for these can be found on page 8 of the data booklet.
N.B. The charge is on the WHOLE GROUP of atoms
The valency is equal to the number of charges.
Always place the formula for the group ion in brackets.

11. Worked Example 1: Calcium Nitrate
Symbol Valency Swap
Ca(NO3)2

12. Example 2: Ammonium carbonate
NH4 CO3 1 2 (NH4)2(CO3)1 (NH4)2CO3
Brackets can be removed ONLY if the subscript is 1

13. Further Examples: Magnesium hydroxide Ammonium phosphate
Potassium chromate
Aluminium sulphate
Calcium hydrogensulphite
Mg(OH)2 2. (NH4)3PO4 3. K2CrO4
4. Al2(SO4)3 5. Ca(HSO3)2

14. Mixed Questions: Yellow Book Page 29 Q 12 Int 2 Textbook Page 64 Q11
Group ion Practice: Yellow Book Page 29 Q 11 Int 2 Textbook Page 61 Q5&6
Mixed Questions:
Yellow Book Page 29 Q 12
Int 2 Textbook Page 64 Q11

15. Formula from Prefixes
In some cases the name of the compound tells us its chemical formula. This information is given in the form of a prefix.
If only one prefix is given it can be assumed that only one atom of the other element is present.
N.B. You do not need to ‘cross over’ if a prefix is given.

16. Prefix
Meaning
Mono- 1
Di- 2
Tri- 3
Tetra- 4
Penta- 5
Hexa- 6
Hepta- 7

17. Worked Examples Carbon monoxide: CO Carbon dioxide: CO2
Dinitrogen monoxide: N2O
Phosphorus trichloride: PCl3

18. Further Examples Diphosphorus trioxide Silicon tetrachloride
P2O3
SiCl4
V2O5
As4O6
UF6
Diphosphorus trioxide
Silicon tetrachloride
Divanadium pentoxide
Tetraarsenic hexaoxide
Uranium hexafluoride