The hydrogen atom has one proton, one electron and no neutrons. This hydrogen atom is also called protium, which is also an isotope. The atomic number of this gas element is 1, and its atomic weight is 1.00794 grams per mole.
More than 90 percent of the universe is made up of hydrogen. Hydrogen plays an important function in nuclear fusion. Hydrogen also is needed by many stars like the sun as a source of energy.
Although protium is one isotope, the two other isotopes of hydrogen are deuterium and tritium. Deuterium has one neutron and the same number of electrons and protons, which is one. Tritium has two neutrons and one each of protons and electrons. Tritium is a radioactive isotope with a half-life of 12 1/2 years.