(a) Isotopes: Isotopes are atoms which have the same number of protons but different number of neutrons. They have same atomic number but different mass number.
Example: Carbon molecule exists as 6C12 and 6C14 but when their electronic configuration is K-2; L-4
(b) Isobars: Isobars are atoms which have the same mass number but different atomic number.
Example: Electronic configuration of 20Ca40is – K-2; L-8; M-8; N- 2 and of 18Ar40 is – K-2; L-8; M-8