Explain the following terms :
Chiral centre (atom) and Achiral molecule

Explanation

Chiral centre : A `sp^(3)` atom bonded with four different groups is called chiral centre or chiral different groups is called chiral centre or chiral atom.
J. Van.t Hoff and C. Le Bel independently argued that the spatial arrangement of four groups around a central atoms of a carbon is tetrahedral and if all the substituents attached to that carbon are different, the mirror image of the molecule is not superimposed on the molecule. Such a carbon is called asymmetric carbon or stereocentre.

Note : The presence or absence of chiral atom is not criteria for a molecule to be optically active. The optical activity is due to molecular asymmetry (Chirality). However, a molecule with one chiral centre is always optically active.
The molecule that lacks a symmetry are called achiral molecules. These are optically inactive.

Propan-2-ol (A) does not contain an asymmetric carbon, as all the four groups attached to the tetrahedral carbon are not different. The mirror image (B) when rotated by `180^(@)` gets completely overlap on (A) and (C ).