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:

- Write the symbols of the two radicals side by side with valencies at the top, the positive radical to the left and negative radical to the right.
- Cross the valencies after removing the common factor, if any. The numbers are to be placed to the lower right of each symbol. The compound radical must be enclosed within a bracket and the number placed outside the bracket to the lower right.

Let's take a few examples:

**Example 1.** For writing the formula of sodium sulphate, proceeding as above instructions state, we have:

- Writing symbols side by side with valencies at the top. Na
^{1+}SO_{4}^{2-} - There being nothing common in 1 and 2, crossing the valencies we get the desired formula: Na
_{2}SO_{4}

More examples coming soon