Atoms that have the same atomic number but different atomic masses are called isotopes. The difference in mass arises due to the atoms containing a different number of neutrons for the same number of protons.
The identity of an element is determined by the number of protons each of its nuclei contains. Protons are positively charged and held together in a small space. The electrostatic force of repulsion between the like charges is very high, but they are able to remain stable due to the presence of neutrons. Neutrons are neutral subatomic particles found within the nucleus of an atom. The protons and neutrons are the heaviest subatomic particles and constitute the entire atomic mass of an element.
When two atoms have the same number of protons, they are said to be from the same element. However, if they differ in atomic mass, it means that the number of neutrons in the nuclei is different for the two atoms. Since the number of neutrons does not change the electron configuration of the atoms, the two atoms have the same chemical properties. However, they have different physical properties, such as boiling point, melting point and density, because their atomic masses are different.
To account for the similarities in chemical properties but the difference in physical properties, the two atoms of the same element are called isotopes of that element. For example, hydrogen has two other isotopes, deuterium and tritium, caused by the addition of extra neutrons in the nucleus without a change in the number of protons or electrons.