Hydrogen | Properties, Uses, & Facts | Britannica

Physical and chemical properties

The Table lists the important properties of molecular hydrogen, H2. The extremely low melting and boiling points result from weak forces of attraction between the molecules. The existence of these weak intermolecular forces is also revealed by the fact that, when hydrogen gas expands from high to low pressure at room temperature, its temperature rises, whereas the temperature of most other gases falls. According to thermodynamic principles, this implies that repulsive forces exceed attractive forces between hydrogen molecules at room temperature—otherwise, the expansion would cool the hydrogen. In fact, at −68.6° C attractive forces predominate, and hydrogen, therefore, cools upon being allowed to expand below that temperature. The cooling effect becomes so pronounced at temperatures below that of liquid nitrogen (−196° C) that the effect is utilized to achieve the liquefaction temperature of hydrogen gas itself.

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normal hydrogen	deuterium
Atomic hydrogen
atomic number	1	1
atomic weight	1.0080	2.0141
ionization potential	13.595 electron volts	13.600 electron volts
electron affinity	0.7542 electron volts	0.754 electron volts
nuclear spin	1/2	1
nuclear magnetic moment (nuclear magnetons)	2.7927	0.8574
nuclear quadrupole moment	0	2.77(10−27) square centimetres
electronegativity (Pauling)	2.1	~2.1
Molecular hydrogen
bond distance	0.7416 angstrom	0.7416 angstrom
dissociation energy (25 degrees C)	104.19 kilocalories per mole	105.97 kilocalories per mole
ionization potential	15.427 electron volts	15.457 electron volts
density of solid	0.08671 gram per cubic centimetre	0.1967 gram per cubic centimetre
melting point	−259.20 degrees Celsius	−254.43 degrees Celsius
heat of fusion	28 calories per mole	47 calories per mole
density of liquid	0.07099 (−252.78 degrees)	0.1630 (−249.75 degrees)
boiling point	−252.77 degrees Celsius	−249.49 degrees Celsius
heat of vaporization	216 calories per mole	293 calories per mole
critical temperature	−240.0 degrees Celsius	−243.8 degrees Celsius
critical pressure	13.0 atmospheres	16.4 atmospheres
critical density	0.0310 gram per cubic centimetre	0.0668 gram per cubic centimetre
heat of combustion to water (g)	−57.796 kilocalories per mole	−59.564 kilocalories per mole

Hydrogen is transparent to visible light, to infrared light, and to ultraviolet light to wavelengths below 1800 Å. Because its molecular weight is lower than that of any other gas, its molecules have a velocity higher than those of any other gas at a given temperature and it diffuses faster than any other gas. Consequently, kinetic energy is distributed faster through hydrogen than through any other gas; it has, for example, the greatest heat conductivity.

A molecule of hydrogen is the simplest possible molecule. It consists of two protons and two electrons held together by electrostatic forces. Like atomic hydrogen, the assemblage can exist in a number of energy levels.

Ortho-hydrogen and para-hydrogen

Two types of molecular hydrogen (ortho and para) are known. These differ in the magnetic interactions of the protons due to the spinning motions of the protons. In ortho-hydrogen, the spins of both protons are aligned in the same direction—that is, they are parallel. In para-hydrogen, the spins are aligned in opposite directions and are therefore antiparallel. The relationship of spin alignments determines the magnetic properties of the atoms. Normally, transformations of one type into the other (i.e., conversions between ortho and para molecules) do not occur and ortho-hydrogen and para-hydrogen can be regarded as two distinct modifications of hydrogen. The two forms may, however, interconvert under certain conditions. Equilibrium between the two forms can be established in several ways. One of these is by the introduction of catalysts (such as activated charcoal or various paramagnetic substances); another method is to apply an electrical discharge to the gas or to heat it to a high temperature.

The concentration of para-hydrogen in a mixture that has achieved equilibrium between the two forms depends on the temperature as shown by the following figures:

Essentially pure para-hydrogen can be produced by bringing the mixture into contact with charcoal at the temperature of liquid hydrogen; this converts all the ortho-hydrogen into para-hydrogen. The ortho-hydrogen, on the other hand, cannot be prepared directly from the mixture because the concentration of para-hydrogen is never less than 25 percent.

Key People: Henry Cavendish Antoine Lavoisier Otto Struve Anders Jonas Ångström Fritz Wolfgang London (Show more) Related Topics: hydride deuterium tritium protium Pfund series (Show more) On the Web: Frontiers - Frontiers in Energy Research - Impacts of hydrogen on tropospheric ozone and methane and their modulation by atmospheric NOx (Jan. 16, 2026) (Show more) See all related content

The two forms of hydrogen have slightly different physical properties. The melting point of para-hydrogen is 0.10° lower than that of a 3:1 mixture of ortho-hydrogen and para-hydrogen. At −252.77° C the pressure exerted by the vapour over liquid para-hydrogen is 1.035 atmospheres (one atmosphere is the pressure of the atmosphere at sea level under standard conditions, equal to about 14.69 pounds per square inch), compared with 1.000 atmosphere for the vapour pressure of the 3:1 ortho–para mixture. As a result of the different vapour pressures of para-hydrogen and ortho-hydrogen, these forms of hydrogen can be separated by low-temperature gas chromatography, an analytical process that separates different atomic and molecular species on the basis of their differing volatilities.