Elements have different combination powers. For example 1 atom of oxygen combine with 2 atoms of hydrogen to form water, but 1 atom of chlorine can only combine with 1 atom of hydrogen. As such oxygen may to have a combining number of 2, while hydrogen have a combining number of 1. The number is sometimes refer to valency of an elements, but note that valency is also used to describe how atom combine and as well as how many number combination take place.
To check the formula of a compound:
A symbol represent ONE ATOM of an element.
the formula of a compound represents the simplest ratio of atoms in a compound.*
Many simple compound is made up of two part-metal metallic and non metallic. The sum of the combining power of one side must be equal to the sum of combination with the other side.
Example 1. Check the formula of aluminum oxide.
Metal 1 atom of aluminium, Al, have 3 combining number.
Non metal 1 atom of oxygen, O, have 2 combining number.
2 atom of aluminium, Al2, the combining power is = 3 x 2 = 6.
3 atom of oxygen O3, the combining power is = 2 x 3 = 6.
The combining power of Al2 = the combining power of O3.
Therefore the simplest formula of aluminium oxide is = Al2O3.