The hydrogen atom is both the most abundant element in the universe (75% by mass and 92% by number of atoms) and a very rare gas in its “pure” form, dihydrogen. It’s common use to talk about the “hydrogen molecule”, so we will use this terminology.

Hydrogen has different faces, physical aspects, which we are going to develop here.

This article is a part of our dossier on hydrogen, innovation and ecology.

The states of the hydrogen molecule

A hydrogen atom is made up of one proton and one electron. Its atomic mass is 1.00794u.

Hydrogen becomes liquid at -253°C and solid at -262°C.

The hydrogen molecule in its “normal” state: the gas

What is called, in everyday language, hydrogen, is dihydrogen (that is to say the molecule formed of two hydrogen atoms). As a gas, its density is 0.08988g/L. To be used, it needs to be compressed (otherwise, 1 kg of hydrogen occupies 11,000 liters!). On compression in general at 200 bars for transport and at 350 or 700 bars for hydrogen vehicles. Here are some orders of magnitude:

  • Hydrogen density 200 bars: ~14 g/L, i.e. 71.4 liters to contain 1 kg of hydrogen
  • Hydrogen density 350 bars: ~21 g/L, i.e. 47.6 liters to contain 1 kg of hydrogen
  • Hydrogen density 700 bars: ~42 g/L, i.e. 23.8 liters to contain 1 kg of hydrogen

Liquid hydrogen

Hydrogen becomes liquid at 20.28 degrees Kelvin (= −252.87 °C) at atmospheric pressure. Its density then goes from 0.08988 g/l to 70.973 g/l.

It is the densest form of dihydrogen storage, 1.5 times less voluminous than its competitor, gaseous hydrogen at 700 bars. If it is only used for space conquest, it could find applications in heavy mobility.

To go further, you can consult our article on liquid hydrogen.

Metallic Hydrogen

Hydrogen can also be “metallic”, under certain extreme conditions. Metallic hydrogen was first “created” in 2020. It is produced at pressures above 425 gigapascals. It is the material that could store the most energy and be superconducting… at room temperature.

Its density is 70.6g/L in solid form.

To go further, you can consult our article on metallic hydrogen.

Hydrogens isotopes

The hydrogen atom has several isotopes (hydrogens whose nucleus contains different numbers of neutrons). The “normal” hydrogen, also called “protium” as 0 neutron. The deuterium (or medium-heavy hydrogen) gas 1 neutron and tritium (heavy hydrogen) has 2 neutrons.

For more details on this point, I refer you to the article on the isotopes of hydrogen.

The names of hydrogen: how is he made ?

Dihydrogen has several names depending on its mode of production.

  • Green hydrogen is made from electolyse powered with renewables energies.
  • Brown hydrogen is produced with fossil fuels, through vaporeforming of gazeification for example.
  • Fatal hydrogen is produced as a byproduct of another process. It’s notably produced through coke or chlorine productions.
  • Natural or native hydrogen comes from deposits undeground. I’m not aware of existing facilities.