Deuterium: Definition, Mass & Density

Instructor: Jen Look

Jen has a doctorate in chemistry and has more than a decade of experience as an educator.

Deuterium is a specific type of hydrogen atom. Also known as 'heavy hydrogen' this form of hydrogen contains an extra particle, giving it a higher mass and density than most hydrogen atoms.

What is deuterium?

Deuterium is a specific type of hydrogen that has more mass than normal. Technically, we say deuterium is an isotope of hydrogen.

What is an isotope?

To understand this, we need to think about the structure of atoms. Atoms are made of 3 particles: protons, neutrons, and electrons. The number of protons (positively charged particles) determines what type of atom we have, which we can find on the periodic table.

periodic table

Every atom that has 5 protons is a boron atom, and every gold atom in the whole universe has 79 protons. Neutrons are neutral particles which influence how an atom behaves, but not its identity. We call an atom with a specific number of neutrons an isotope, and we name isotopes by adding the number of protons and neutrons together to get the mass number, which we stick on the end of the name. So an atom with 8 protons and 8 neutrons would be the isotope oxygen-16, while isotope oxygen-12 has 8 protons plus 4 neutrons. When writing the symbols rather than names, we write the mass number first as a superscript, but we still pronounce it as 'oxygen twelve.'


The number of electrons (the negatively charged particles) determines the ionic charge. Since the electrons are on the outside of the atom, the number of electrons is easy to change. Changing the number of protons or neutrons requires a nuclear reaction--not something we do in most chemistry experiments!

This handy table summarizes what you need to know about the 3 subatomic particles:

Subatomic particle summary

Deuterium Symbol & Explanation

So now we can actually understand deuterium. Hydrogen is the smallest type of atom: it has only one single proton. The two most common isotopes of hydrogen are hydrogen-1 (1 proton, 0 neutrons), and hydrogen-2 (1 proton, 1 neutron). Because hydrogen-2 is really important, scientists gave it an additional name, deuterium. The official symbol is ^2H, but people often use the informal symbol D to represent deuterium. Protons and neutrons have similar masses, so the mass of a deuterium particle is twice as much as a hydrogen-1 particle, giving it a mass of approximately 2 atomic mass units (amu). This leads to its nickname, heavy hydrogen.

Both hydrogen-1 and hydrogen-2 occur in nature. When we refer to hydrogen, we really mean the way we find it in nature: with both isotopes jumbled together. They aren't found in equal amounts, however. There's a lot more hydrogen-1 than deuterium. If we have a sample with a million hydrogen atoms, we would have only about 100 atoms of deuterium and the remaining 999,900 would all be be hydrogen-1. Several heavier isotopes of hydrogen have been made in laboratories, but they are unstable and do not exist in nature.

Hydrogen isotopes

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