Formulae are written using symbols and valencies of elements and radicals.

Steps in writing chemical formula

  1. Write the symbols of the elements and formula of the radicals involved
  2. Write the valencies of the species in the top right-hand corner.
  3. Reverse the valencies and write them on the bottom right-hand corner.

Never write 1 in the chemical formula.


(a) To write the formula of calcium chloride

Step1               Ca        Cl

Step2               Ca2      Cl1

Formula             CaCl2

(b) To write the formula of magnesium carbonate.

                  Mg        CO3

                  Mg2       CO32

                       Mg2 (CO3)2


( c )To write the formula of Zinc nitrate

                   Zn        NO3

                            Zn2      NO31


(d) To write the formula of carbondioxide

                  C                O

                  C4              O2


 (e) To write the formula of water.

                        H             O

                        H1        O2



Write the chemical formulae of the following compounds:

  1. Sodium Carbonate
  2. Ammonium Sulphate
  3. Copper (II) hydroxide
  4. Sodium Oxide
  5. Calcium hydrogen carbonate
  6. Potassium hydrogen carbonate
  7. Ammonium Phosphate
  8. Calcium Phosphate
  9. Iron (III) Sulphate

10. Aluminium Sulphate