The most common isotope of hydrogen is protium, which has one proton and no neutrons in its nucleus. Protium is the simplest and lightest isotope of hydrogen, with an atomic mass of approximately one atomic mass unit (amu). This isotope makes up about 99.98% of all naturally occurring hydrogen. Hydrogen's other two isotopes are deuterium, which has one neutron, and tritium, which has two neutrons and is radioactive. Protium plays a significant role in various chemical reactions and is a fundamental component of water and organic compounds, making it essential to life as we know it.