How to write the chemical formula of compounds?

You have to remember the valency of these acid & basic radicals then you can easily write the formula of any compound.

Eg - Hydrogen    Sulphate =   H2SO4
              +1          ×     -2
    (Monovalent)  (Divalent)
   
         Aluminum    Chloride  =  AlCl3
               +3           ×       -1