Hydrogen is the atomic particle that has the least mass. An atom is made of a combination of subatomic particles known as protons, neutrons and electrons. The combined mass of these particles determines the mass of the atom. Hydrogen has the least number of subatomic particles, thus the least mass.
A common hydrogen atom contains just one proton and one electron. Another form of hydrogen atom, known as deuterium, contains one neutron as well as a proton and an electron. A third form of hydrogen, known as tritium, contains two neutrons along with one proton and one electron. These less-common forms of hydrogen are known as isotopes. An isotope contains the same number of electrons and protons as the common form of an atom but has a different number of neutrons.