What are Isotopes?

Atoms and elements are made of protons, neutrons and electrons. The nucleus is made of protons and neutrons, and the electrons surround the nucleus, as shown in the illustration below. The sum of the number of protons and the number of neutrons is equal to the atomic mass.
Isotopes are atoms of the same element possesing the same number of protons and a different number of neutrons.In an element the atomic number is equal to the number of protons and the atomic mass is the sum of protons and neutrons
Thus, Isotopes are elements which have the same atomic number but a different mass number from their counterparts.
Let us take a look at an example...
tHydrogen has three isotopes. They all have the same atomic number, or number of protons, (number at the lower left of the element) but different atomic masses (number at the upper left of the element).
The number of neutrons can be calculated by calculating the difference between the atomic mass and atomic number. We can see that for the isotopes of hydrogen, they have varying number of neutrons. For protium, the number of neutrons is zero; for deuterium, the number of neutrons is one; and for tritium, the number of neutrons is two.
Thus, we can say that these three isotopes of hydrogen are like identical triplets of each other - they may appear to be identical outside, but they are different inside, and they also have different names.