Chemical Formula

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.

Examplesoxygen moleculecontains 2 atoms O2

bromine moleculecontains 2 atoms Br2

nitrogen moleculecontains 2 atoms N2

If the formula contains only one atom of a particular type, there is no need to put in ‘1’.

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

1. Hydrogen fluoride2. Carbon chloride3. Phosphorus hydride4. Sulphur oxide

HFCCl4PH3

SO

Valency Method

The valency of an atom depends on how many of its electronsare involved in bonding.

Remember:Group number Number of bonds can form (valency)

1 1

2 2

3 3

4 4

5 3

6 2

7 1

Valencies can be used to work out formula.

Step 1: Write down the symbols for the elements presente.g. calcium cholride

Ca Cl

Step 2: write the valencies of the elements present above each symbol

Ca Cl

2 1

Step 3: Cross over the valencies

Ca Cl2 1

Ca1Cl2

Step 4: if there are any common factors cancel them out and omit any ‘1’ present

The formula of calcium chloride is: CaCl2

What about silicon oxide:

Step 1: Write down the symbols for the elements present

Si O

Step 2: write the valencies of the elements present above each symbol

Si O

4 2

Step 3: Cross over the valencies

4 2Si O

Si2O4

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 SiO2.

Examples:

•Phosphorus chloride•Boron sulphide•Silicon fluoride•Sulphur chloride•Phosphorus iodide•Iodine bromide•Boron chloride

PCl3B2S3

SiF4

SCl2PI3IBrBCl3

The same method works for compounds containing metals and non metals.

Try these examples:

•Magnesium oxide•Sodium chloride•Aluminium iodide•Potassium sulphide•Lithium oxide

MgONaClAlI3K2SLi2O

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.

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) oxide2. Tin(IV) chloride3. Iron(II) oxide4. Iron(III) sulphide

Cu2OSnCl4FeOFe2O3

Transition metal’s name

Transition metal’s symbol

valency

Copper(II)    

Copper(I)    

Tin(IV)    

Silver(I)    

Lead(V)    

Iron(III)    

Using Prefixes

In some cases the compounds name helps you to findthe formula.

prefix meaning

mono one

di two

tri three

tetra four

penta five

hexa six

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      

Examples

•Carbon monoxide mono = 1 one oxygen: CO

•Carbon dioxide di = 2 two oxygens: CO2

•Phosphorus pentachloride pent = 5

five chloride atoms: PCl5