What is isotopes?

Dear Student,
Answer to your question is given below:
Isotopes are defined as atoms of same element having the same atomic number, but different mass numbers. These atoms contain an equal number of protons and electrons, but a different number of neutrons.

For example, in nature, hydrogen is found in three forms with different mass numbers, namely protium (), deuterium(), and tritium (). The atomic number of each isotope of Hydrogen is 1, but the mass number varies. 
Regards,