In nature, the vast majority of hydrogen atoms have no neutrons; these atoms consist of one electron and one proton only, and are the lightest atoms possible. However, rare isotopes of hydrogen, called deuterium and tritium, do have neutrons. Deuterium has one neutron, and tritium, unstable and not seen in nature, has two.
TL;DR (Too Long; Didn't Read)
Most hydrogen atoms have no neutron. However, rare isotopes of hydrogen, called deuterium and tritium, have one and two neutrons each, respectively.
Elements and Isotopes
Most elements in the periodic table have several isotopes – “cousins” of the element that have the same number of protons but different numbers of neutrons. Isotopes appear very similar to one another and have similar chemical properties. For example, alongside the abundant carbon-12 isotope, you can find tiny amounts of radioactive carbon-14 in virtually all living things. However, because neutrons have mass, the weights of isotopes are slightly different. Scientists can detect the difference using a mass spectrometer and other specialized equipment.
Uses for Hydrogen
Hydrogen is the most abundant element in the universe. On Earth you’ll seldom find hydrogen by itself; much more often it is combined with oxygen, carbon and other elements in chemical compounds. Water, for example is hydrogen joined with oxygen. Hydrogen plays an important role in hydrocarbons, such as oils, sugars, alcohols and other organic substances. Hydrogen also serves as a “green” energy source; when burned in air; it gives off heat and pure water without producing CO2 or other harmful emissions.
Uses for Deuterium
Although deuterium, also known as “heavy hydrogen,“ occurs naturally, it is less abundant, accounting for one out of every 6,420 hydrogen atoms. Like hydrogen, it combines with oxygen to produce “heavy water,” a substance that looks and behaves much like ordinary water, but which is slightly heavier and has a higher freezing point, 3.8 degrees Celsius (38.4 degrees Fahrenheit), compared to 0 degrees Celsius (32 degrees Fahrenheit). The extra neutrons make heavy water useful for radiation shielding and other applications in scientific research. Being rare, heavy water is also much more expensive than the ordinary kind. Its extra weight makes it chemically somewhat odd compared to water. At normal concentrations, it’s nothing to worry about; however, amounts over 25 percent will damage the blood, nerves and liver, and very high concentrations can be deadly.
Uses for Tritium
The extra two neutrons found in tritium make it radioactive, decaying with a half-life of 12.28 years. Without a natural supply of tritium, it must be made in nuclear reactors. Although its radiation is somewhat hazardous, in small amounts and with careful handling and storage, tritium can be beneficial. “Exit” signs made with tritium produce a soft glow that remains visible for up to 20 years; because they don’t need electricity, they provide safety lighting during power blackouts and other emergencies. Tritium has other uses in research, such as tracing the flow of water; it also plays a role in some nuclear weapons.