Question

what are isotopes explain by giving an example

Answer 1

each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties

Carbon 12 and Carbon 14 are both isotopes

¹₆²C      &        ¹₆⁴C

Answer 2

Here's your answer mate.

Isotopes are atoms of the same element having same atomic number but different mass numbers.

Examples:
All chlorine atoms contains 17 protons but some chlorine atoms contain 18 neutrons whereas the other one contains 20 neutrons.
Thus,Chlorine atoms can have two mass numbers:
17+18=35
17+30=37
Therefore,Chorine has two isotopes of mass number 35 and 37.

Hope it helps you.