In figure below, we show three different types of hydrogen atoms which contain one proton in the nucleus but the number of neutrons is different.
The atoms in most commonly found hydrogen contain no neutrons in their nuclei. The next most commonly found hydrogen atom is called deuterium. It contains one neutron in addition to a proton in its nucleus.
The other type of atom of hydrogen rare than deuterium, contains two neutrons in addition to a proton in its nucleus. It is called tritium. One could describe the three types of hydrogen atoms along with their atomic and mass numbers as 1H1, 1H2, and 1H3.
Since the chemical properties of an element are determined by the atomic number (Z) of the atom of that element, hence all three atoms of hydrogen, behave chemically in the same manner.
Atoms of an element having the same atomic number but different mass number are called isotopes of that element.
Since the mass number is determined jointly by the sum of the number of protons and neutrons in the nucleus, hence each isotope of an element will have the same number of protons but different number of neutrons in the nuclei.