Structural isomers are chemical compounds that have

Answer- The answer to the given question is similar molecular formula.

Explanation-

Certain compounds have the same formula because they have the same number of atoms and substances involved and are called isomers. The only difference between the two isomers is the structural arrangement of these atoms they have.  

The formation of the structure of atoms being different it makes the substances different from each other. A simple example could be isobutane and butane having same formula but different structures.