Isomerism –
“When two or more compounds having the same molecular formula but differ in physical or chemical properties are known as isomers and the phenomenon is called isomerism.”
The term isomer was given by Berzelius ( Greek word ; Iso means equal , meros means parts)
source : ProProfs
Structural isomerism-
When two or more compounds having the same Molecular Formula but different structures i-e. different arrangement of atoms or group of atoms within the molecule , the phenomenon is known as structural isomerism and such compounds are called structural isomers.
Types of structural isomerism-
a) Chain isomerism – It is also known as nuclear isomerism or skeletal isomerism.When two or more compounds having the same Molecular Formula but differ in the arrangement of Carbon atoms in the main chain , the phenomenon is known as chain isomerism and such compounds are called chain isomers.
Ex 1) C4H9Br – two chain isomers
CH3-CH2-CH2-CH2Br & 
1- bromo butane 1- bromo -2- methyl propane
2) C4H10 – Two chain isomers are n- butane & iso-butane (or 2- methyl propane)
3) C5H12- Three chain isomers are n – pentane , iso pentane (or 2- methyl butane ), neo – pentane ( or 2,2 – dimethyl propane)
4) C4H8 – Two chain isomers are butene -1 & 2- methyl propene.
5) C4H10O – Two chain isomers are n- butyl alcohol ( or butanol -1) & 2 – methyl propan – 1 – ol
6) C4H11N- Two chain isomers are Butanamine – 1 & 2- methyl propanamine -1
