What makes isotopes of the same element differ from each other?There can be many isotopes of the same element. What makes isotopes different?

The atom is made up of three major particles.  Protons and
neutrons which make up the nucleus and electrons which surround the nucleus.  It is the
number of protons, or the atomic number, the one that makes an element what it is.  If
you change the number of protons, you no longer have that element.  The other nuclear
particle is the neutron. The mass number is a sum of the number of protons, or atomic
number, and the number of neutrons.  It is these particles which give an atom its mass,
the electrons do not weigh much and their contribution to the mass is negligible.  If
you change the number of neutrons, the mass number (which is a sum of protons and
neutrons) changes, but the atomic number remains the same. This is how you can determine
if an atom is an isoptope of the same element.  The mass number is different, but the
atomic number is the same.

Example:  Hydrogen has 3
isotopes: Protium: atomic number=1, mass number= 1; Deuterium: atomic number= 1, mass
number= 2; and Tritium: atomic number=1, mass number = 3.