Hydrogen isotopes are variations of the hydrogen atom that differ in the number of neutrons in their nuclei. The three main isotopes are protium, which has no neutrons, deuterium, which has one neutron, and tritium, which has two neutrons. These isotopes have similar chemical properties but differ in mass and stability. Protium is the most abundant isotope, making up about 99.98% of natural hydrogen. Deuterium is found in small amounts in water, while tritium is radioactive and occurs in trace amounts. The study of hydrogen isotopes is important in fields like nuclear fusion and environmental science.