How Many Neutrons Does Hydrogen Have?

The proton and the electron were well documented during beginning of the 20th century, but scientists knew there was some unknown influence or particle within the nucleus. This is because the atomic number (the number of protons) was always around half the atomic mass number for any atom. This indicated some other particle, similar in mass to the proton, found in similar quantities to the number of protons in an atom. In 1932, English physicist James Chadwick confirmed the existence of this particle: the neutron.

An atomic number denotes the number of protons in the nucleus of an atom on the periodic table. With stable, neutral atoms, they will have an equal number of protons and electrons – resulting in a net zero charge.

Atomic mass units (or atomic weight units) are then used to measure the mass of atoms. One atomic mass unit (amu) is defined as 1/12 the mass of a Carbon-12 atom. A carbon-12 atom has 6 protons and 6 neutrons, so the mass of a proton or a neutron works out to be approximately 1 amu.

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.

The element hydrogen comes in three stable isotopes found on Earth (there are other unstable isotopes that only last for less than a quintillionth of a second before decaying). These are protium, deuterium, and tritium.

\(\begin{align*}\)
\(&{}^1 \text{H} \rightarrow \text{zero neutrons}\)
\(&{}^2 \text{H} \rightarrow \text{one neutron}\)
\(&{}^3 \text{H} \rightarrow \text{two neutrons}\)
\(\end{align*}\)

All of these hydrogen isotopes have the same neutral charge and the same number of electrons (to balance the positive charge from the proton).

The chemical element hydrogen is the lightest and most abundant element in the universe. Protium (with zero neutrons) is the most common isotope of hydrogen.

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.

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.

The extra two neutrons found in tritium make it a radioactive isotope, 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.