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.
A chemical reaction is marked by new substance formation. The energy changes in chemical reactions occur when atoms <u>rearrange</u> to form new substances.
<h3>What is a chemical reaction?</h3>
A chemical reaction is said to be the process that is characterized by the formation of new substances by the alteration of the chemical composition and the properties of the compound or the molecule.
The reaction results in a new product by the rearrangement of the atoms of the reactant molecule by breaking and creating chemical bonds. The bonds are altered by the change in energy.
Therefore, <u>rearrangement </u>is the correct blank.
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