In the formation of a chemical compound, combinaton takes place between positive and the negative radicals in such a way that the product of valency and the number of radicals is the same for both the radicals. Thus to write the formula of a compound we proceed as follows: Let's take a few examples: Example 1. For writing the formula of sodium sulphate, proceeding as above instructions state, we have: 1. * Writing symbols side by side with valencies at the top. Na1+SO42- 2. * There being nothing common in 1 and 2, crossing the valencies we get the desired formula: Na2SO4