Hydrogen has no neutrons, just a single proton and a single electron. Most of the mass of an atom lies in its protons and neutrons; hydrogen has a mass of 1 because of the mass of its single proton.
Hydrogen has three common isotopes: protium, deuterium and tritium. Isotopes have the same atomic number, meaning they are the same element because they have the same number of protons and electrons; however, their masses differ because they have varying numbers of neutrons. Protium has no neutrons, deuterium has one neutron, and tritium has two neutrons. The most common of the isotopes is protium, which makes up 99.9 percent of the hydrogen in nature. Heavy water, which is denser and heavier than normal water, consists of hydrogen in the form of deuterium. Tritium is radioactive and is formed in the atmosphere.