Isomers:
 The phenomenon of exhibiting different physical and chemical properties even though having same molecular formula is called isomerism and the molecules showing isomerism are called isomers.
 Isomers are organic compounds which have the same molecular formula but different structural formula.
 Features of Isomers:

1. They have same molecular formula

2. They have different structural formula.

3. They vary in their chemical properties.

4. They have different boiling and melting points.

Isotopes:
 The atoms which posses same atomic number (the atoms same element) but different mass number are called Isotopes.

The difference in mass number is due to different number of neutrons present in the nucleus.
 Ex: ₁H¹, ₁H², ₁H³

These are the isotopes of Hydrogen.

These isotopes can show similar chemical properties as they are of same of element hydrogen.
 
 Isobars:

Atoms of different elements with different atomic numbers but have the same mass number are called isobars.

Example: 18Ar40, 19K40 and 20Ca40.