How to write a Chemical Formula for Ionic Compounds with examples

Compounds are represented by chemical formulae as elements are represented by symbols.chemical formulae of compounds are written keeping the following steps in consideration.

  • Symbols of two elements are written side-by-side, in the order of positive ion and negative ion latter.
  • The valency of each ion is written on the right top corner of its symbol, e.g,Na+,Ca2+, Cl and O2-
  • The valency of each ion is brought to the lower right corner of other ion by cross-exchanging method, e.g.


Na+ Cl       Ca2+Cl     and      Ca2+O2-

They are written as:

NaCl         CaCl2        and        CaO

  • If the valencies are same, they are offset and are not written in the chemical formula. But if they are different, they are indicated as such at the same position, e.g. In case of sodium chloride is represented by NaCl, whereas calcium chloride is represented by formula CaCl2
  • If an ion is a combination of two or more atoms which is called radical, bearing a net charge on it, e.g. SO42-.(sulphate), and PO43- (phosphate), then the net charge represented by valency of the radical. The chemical formula of such compounds is written as explained in (iii) and (iv); writing the negative radical within the parenthesis. For example, chemical formula of aluminium sulphate is written as Al2(SO4)3 and that of calcium phosphate as Ca3(PO4)2.

Significance of chemical formula

  • Chemical Forlmula represents the name of the substance e.g. H2O (water)
  • It also represents one mole of the molecule in the balanced chemical equation.
  • Chemical Forlmula is infact one molecule or formula unit of the compound.
  • It indicates the mass of the copound in amus or grams.
  • Chemical Forlmula tells the name of the elements as present in the compound.

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