How to write a formula?

In the formation of chemical compound, combination takes place between the 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 follow:

            Write the symbols of the two radicals side by side with valencies at the top, the positive radicals 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.

Example1. For writing the formula of sodium sulphate , proceeding as above we have:

            Writing symbols side by side with valencies at the top.

                                  Na¹⁺SO₄^2-

      There being nothing common in 1 and 2, crossing the valencies we get the desired formula:

                                    Na₂SO₄

Example2. To write the formula of ammonium dichromate and proceeding as usual:

                        1^st step: NH₄¹⁺Cr₂O₇^2-

                              2^nd step: (NH₄)₂Cr₂O₇