Chemical Formula

1. Chemical Formula

2. Dictionary:
The chemical formula of a molecule shows the number
of atoms of each element in the molecule.
Formula consists of the symbols for the elements
that make up the molecule, each followed by a number.

3. Examples
oxygen moleculecontains 2 atoms
O
2
bromine moleculecontains 2 atoms
Br
2
nitrogen moleculecontains 2 atomsN
2
If the formula contains only one atom of a particular type,
there is no need to put in ‘1’.

4. to do:
•Draw a target diagram for the following covalent
compounds
•Draw a diagram of the molecule using lines to represent
the covalent bonds
•Write the formula for each compound under the target
diagram
9.Hydrogen fluoride
10.Carbon chloride
11.Phosphorus hydride
12.Sulphur oxide
HF
CCl
4
PH
3
SO

5. Valency Method

6. The valency of an atom depends on how many of its electrons
are involved in bonding.
Remember:
17
26
35
44
33
22
11
Number of bonds
can form
(valency)
Group number

7. Valencies can be used to work out formula.
Step 1: Write down the symbols for the elements present
e.g. calcium cholride
Ca Cl
Step 2: write the valencies of the elements
present above each symbol
Ca Cl
2 1

8. Step 3: Cross over the valencies
Ca Cl
2
1
Ca
1
Cl
2
Step 4: if there are any common factors cancel them
out and omit any ‘1’ present
The formula of calcium chloride is: CaCl
2

9. What about silicon oxide:
Step 1: Write down the symbols for the elements present
SiO
Step 2: write the valencies of the elements
present above each symbol
Si O
4 2

10. Step 3: Cross over the valencies
4 2
Si O
Si
2
O
4
Step 4: if there are any common factors cancel them
out and omit any ‘1’ present
In this case both factors can be divided by 2.
The formula for silicon oxide is SiO
2
.

11. Examples:
•Phosphorus chloride
•Boron sulphide
•Silicon fluoride
•Sulphur chloride
•Phosphorus iodide
•Iodine bromide
•Boron chloride
PCl
3
B
2
S
3
SiF
4
SCl
2
PI
3
IBr
BCl
3

12. The same method works for compounds containing
metals and non metals.
Try these examples:
•Magnesium oxide
•Sodium chloride
•Aluminium iodide
•Potassium sulphide
•Lithium oxide
MgO
NaCl
AlI
3
K
2
S
Li
2
O

13. More than one valency?
Some elements such as transition metals can
have more than one valency.
Chemists use Roman numbers in the names of compounds
to show which valency is to be used.
e.g in a compound copper chloride where the copper has
valency one, the name is written copper(I) chloride.
If the copper is valency two, the name is
written copper(II) chloride.

14. to do:
•Work out the formula for copper(I) chloride and
copper(II) chloride
•Work out the formula for the following transition
metal compounds:
1.Copper(I) oxide
2.Tin(IV) chloride
3.Iron(II) oxide
4.Iron(III) sulphide
Cu
2
O
SnCl
4
FeO
Fe
2O
3

15. Transition metal’s
name
Transition metal’s
symbol
valency
Copper(II)    
Copper(I)    
Tin(IV)    
Silver(I)    
Lead(V)    
Iron(III)    

16. Using Prefixes
In some cases the compounds name helps you to find
the formula.
sixhexa
fivepenta
fourtetra
threetri
twodi
onemono
meaningprefix

17. Compound’s
name
Number of
atoms of first
element
Number of
atoms of
second
element
Formula
carbon
monoxide
     
sulphur
trioxide
     
dinitrogen
tetroxide
     
sulphur
trioxide
     
dinitrogen
monoxide
     
sulphur
dichloride
     
carbon
tetrachloride
     
carbon
disulphide
     
tri-iron nitrate      

18. Examples
•Carbon monoxide mono = 1 one oxygen: CO
•Carbon dioxide
di = 2 two oxygens: CO
2
•Phosphorus pentachloride pent = 5
five chloride atoms: PCl
5