Hydrogen has three isotopes : protium $\left({}^1\mathrm{H}\right)$, deuterium (${}^2\mathrm{H}$ or $\mathrm{D}$) and tritium (${}^3\mathrm{H}$ or $\mathrm{T}$). They have nearly same chemical properties but different physical properties. They differ in

Isotopes are the atoms of the same element with different mass number. They have the same atomic number. So, number of protons and electrons are same, Hence their electronic configuration are same, but number of neutrons are different.

Hydrogen has three isotopes: protium, ${}_1{}^1\mathrm{H}$, deuterium, ${}_1{}^2\mathrm{H}$ or $\mathrm{D}$ and tritium, ${}_1{}^3\mathrm{H}$ or $\mathrm{T}$.  These isotopes differ from one another in respect of the presence of neutrons. Ordinary hydrogen, protium, has no neutrons, deuterium (also known as heavy hydrogen) has one and tritium has two neutrons in the nucleus.