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Chemical formulae


CHEMICAL FORMULAE


  • A chemical formula represents the composition of a molecule of the substance in terms of the symbols of the elements present in the molecule.
  1. While writing the formula of a molecular compound the less electronegative non-metal element is written on the left hand side whereas the more electronegative non-metal element is written on the right hand side.
  2. In naming the molecular compounds, the name of less electronegative non-metal is written as such but the name of more electronegative non-metal is changed to have the ending ‘ide’.
  • When there are more than one atoms of an element in a molecular compound, then ‘the number of atom’ is indicated by using appropriate ‘prefixes’ in the formula.
  • While writing the formula of ionic compounds as a convention, the cation (positive ion) is written on the left hand side and the anion (negative ion) is written on the right had side.
  • The number of cations and anions is adjusted in such a way that the total number of positive valencies of cations becomes equal to the total number of negative valencies of anions

 

Formulae of compounds:

  • The chemical formula of a compound is a symbolic representation of compound which tells about the number of atoms of each element present in the compound.

 

For example:

H2O  2 atoms of Hydrogen and 1 atom of Oxygen

SO­2   2 atoms of Sulphur and 2 atoms of Oxygen

NH3  1 atom of Nitrogen and 3 atoms of Hydrogen

HCl   1 atom of Hydrogen and 1 atom of chlorine

C2H5OH  2 atoms of carbon, 6 atoms of Hydrogen and 1 atom of oxygen

CCl4   4 atoms of chlorine and 1 atom of carbon

 

Writing chemical formula of a molecular compound:

Step 1:

Write the symbols of the elements present in the compound.

Example:

Aluminium chloride

Aluminium [Al]       Chlorine [Cl]

Step 2:

Write the valency of each element.

  (Cross over of valency of combining atoms)