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 different physical characteristics, such as mass. Isotopes are important in various fields, including nuclear energy, where tritium is used in fusion reactions, and environmental science, where deuterium helps trace water movement. Understanding hydrogen isotopes enhances our knowledge of chemical reactions and the behavior of elements in nature.