Which one is not the structural isomer of pentane?

Of the isomeric hexanes, the isomer which can give two mono chlorinated compounds is 

The following compounds $1$and  $2$ with the identical molecular formula, but different structures are called;

Draw the structures of the isomers for bromobutane. 
 

What are the possible isomers of organic compound?

${\mathrm{C}}_5{\mathrm{H}}_{12}$

${\mathrm{C}}_5{\mathrm{H}}_{12}$

Explain the terms with an example: Structural isomerism