The molecule with same molecular formula but different arrangement of atoms is said to be an isomer.
When 2,2-dimethylbutane reacts with chlorine in the presence of light gives three isomers that is
(3-chloro-2,2-dimethylbutane),
(1-chloro-2,2-dimethylbutane) and
(1-chloro-3,3-dimethylbutane).
In above case, the molecular formula of all isomers are same i.e.
but chlorine is arranged in different positions of carbon. Thus, results isomers.
The reaction is shown in the image.