The compounds having similar molecular formula but different arrangement of atoms or groups in space are called isomers and the phenomenon is called as isomerism.
Isomers are of two types: structural isomers and stereo isomers.
Structural isomers are isomers in which molecules with the same molecular formula have different bonding patterns.
Stereoisomers are isomers in which molecules have the same molecular formula and sequence of bonded atoms but differ in the three-dimensional orientations of their atoms in space.
Thus when molecules have the same number of each element but those atoms are arranged differently they are known as Structural isomers.
