What is an isomer?
Before I define what isomers are ,I will give you a simple example, think that you have three circles,each of same color , same radius and same mass. You can arrange the three circles by putting one next to another, or you can arrange the three one overlapping the other. In both the arrangements has same mass , same color but what differs is the arrangement of circles. This defines isomers.
Isomers are molecules with the same chemical formula but different chemical structures. That is, isomers contain the same number of atoms of each element, but have different arrangements of their atoms in space.
A simple example of isomerism is given by propanol: It has the formula
the two isomers have same molecular formula, same mass , but differs in the arrangement of Hydroxyl (OH ) group.