The first one is called butane (or) in the common system it is known as n-butane.
The second one is called 2-methyl propane or iso-butane in the common system.
We find both of these compounds in nature. They have different properties due to the difference in their structures. Compounds of this type which have the same molecular formula but different properties are called isomers.
The phenomenon of possessing same molecular formula but different properties by the compounds is known as isomerism. The compounds that exhibit isomerism are called isomers.
(iso = same; meros = part, i.e., they have same molecular formula)In the above example isomerism is due to the difference in the structures. Therefore, it is called the structural isomerism.