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 [tex]CH_{3}C(CH_{3})_{2}CHClCH_{3}[/tex] (3-chloro-2,2-dimethylbutane), [tex]ClCH_{2}C(CH_{3})_{2}CH_{2}CH_{3}[/tex] (1-chloro-2,2-dimethylbutane) and [tex]ClCH_{2}CH_{2}C(CH_{3})_{2}CH_{3}[/tex] (1-chloro-3,3-dimethylbutane).
In above case, the molecular formula of all isomers are same i.e.[tex]C_{6}H_{13}Cl[/tex] but chlorine is arranged in different positions of carbon. Thus, results isomers.
The reaction is shown in the image.