↪ Isotopes are the atoms of same element which have same atomic number due to having same number of electron but differ with each other in their mass number due to difference in the number of neutrons.

↪ Due to having same number of electrons in their valence shell isotopes have similar chemical properties but they differ with each other in their physical properties due to difference in their atomic masses.

↪ Hydrogen exists in three different isotopes forms.

1.     Protium or Ordinary hydrogen (₁¹H) or (P)

↪ This is the most abundance isotope of hydrogen (99.98%). It has only one electron in its valence shell and one proton at its nucleus. It is most stable isotope if hydrogen.
↪ So its atomic number is 1 and mass number is also 1.

2.     Deuterium or Heavy Hydrogen (₁²H) or (D)

↪ The natural abundance of this type of isotope is 0.014%. It is also called heavy hydrogen and water containing such deuterium isotope is called heavy water. (D₂O)

↪ Deuterium has one electron in its orbit with one proton and one neutron at its nucleus.
↪  So its atomic number is 1 but mass number is 2 i.e. electron = 1, proton = 1 neutron = 1

3.     Tritium or radioactive hydrogen (₁³H) or (T)

↪ The natural abundance of this type of isotope is 7x10^-16 %.

↪ Being radioactive in nature it emits β high speed electron particles. Its half-life is 12.5 years.
↪ Its atomic number is 1 but mass number is 3 i.e., electron = 1, proton = 1 and neutron = 2