Every chemical compound has a fixed chemical formula. When you are giving the names of a compound, write its formula using these rules:

1. Write down all the symbols of the boding elements and/or radicals in the compound. The symbol of the metal in the compound goes on the left hand side of the formula.
2. Determine and write down the valencies of the bonding elements or radicals below each symbol

1. Two valency numbers cancel each other if they are the same
2. If two valency numbers have a common factor they can be simplified by dividing by that factor.

3. Interchange their valencies and write down the number as a subscript after the element or radical symbol.
4. The value one (1) is not written in a chemical formula.
5. If the subscript of the radical is equal or greater than 2, place brackets around the radical in the formula of the compound.

Example

Write the formula for carbon dioxide

Name of compound                                    carbon dioxide

Component element                      carbon                     oxygen

Symbol of elements                              C                             O

Valency                                                    4                             2

Interchange valencies                             2                           4

Simplify                                                      1                           2

Formula number as subscript                                CO₂

• Atomicity

Atomicity is the number of atoms in a single molecule of an element.

Examples;

1. The formulae of an element with one atom include; He,Li etc
2. The formulae of an element with two atoms include; H2,
3. The formulae of an element with three atoms include; Ozone(O3)

• Valency (valencies)

A Valency is the combining power of an element

The valencies of some elements of the periodic table

Note; An element having a valency of zero means that the element can’t take part in a chemical reaction hence has no charge or it is neutral.

Valency of radicals

A Radical is a group of atoms that behaves as a single particle.