What is geometrical isomerism? What is its cause? Give examples.

When the same molecular formula represents two compounds which differ in the spatial arrangement of atoms or groups around carbon-carbon double bond, then such isomers are called geometrical isomers. The phenomenon is called geometrical isomerism. 
Cause of geometrical isomerism: Geometrical isomerism is due to the restricted or hindered rotation around the carbon-carbon double bond. Due to the hindered rotation around carbon-carbon double bond, the relative positions of atoms or group attached to the doubly bonded carbon atoms get fixed. For example, the molecular formula C₂A₂B₂with the structural formula BAC=CAB represents two geometrical isomers.

Isomer in which similar atoms or groups lie on the same side of the double bond is called cis-isomer. The geometrical isomer I represent a cis-isomer. Isomer in which the similar atoms or groups lie on the opposite sides of the double bond is called trans-isomer. The geometrical isomer (ii) represents a trans-isomer. Hence, geometrical isomerism is known as cis-trans isomerism e.g.
(i) But-2-ene exhibits geometrical isomerism.


(ii) Hex-3-ene shows geometrical isomerism.

(iii) 1, 2-dichloroethene exhibits geometrical isomerism.