Explosion of a hydrogen–air mixture.
Hydrogen gas (dihydrogen or molecular hydrogen)
 is highly flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume.
enthalpy of combustion is −286 kJ/mol:
- 2 H2(g) + O2(g) → 2 H2O(l) + 572 kJ (286 kJ/mol)
Hydrogen gas forms explosive mixtures with air in concentrations from 4–74% and with chlorine at 5–95%. The explosive reactions may be triggered by spark, heat, or sunlight. The hydrogen
autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C (932 °F).
hydrogen-oxygen flames emit
ultraviolet light and with high oxygen mix are nearly invisible to the naked eye, as illustrated by the faint plume of the
Space Shuttle Main Engine, compared to the highly visible plume of a
Space Shuttle Solid Rocket Booster, which uses an
ammonium perchlorate composite. The detection of a burning hydrogen leak may require a
flame detector; such leaks can be very dangerous. Hydrogen flames in other conditions are blue, resembling blue natural gas flames.
destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a rich mixture of hydrogen to oxygen combined with carbon compounds from the airship skin.
H2 reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with
fluorine to form the corresponding hydrogen halides,
hydrogen chloride and
hydrogen fluoride, which are also potentially dangerous
Electron energy levels
Depiction of a hydrogen atom with size of central proton shown, and the atomic diameter shown as about twice the
radius (image not to scale)
energy level of the electron in a hydrogen atom is −13.6
 which is equivalent to an ultraviolet
photon of roughly 91
The energy levels of hydrogen can be calculated fairly accurately using the
Bohr model of the atom, which conceptualizes the electron as "orbiting" the proton in analogy to the Earth's orbit of the Sun. However, the atomic electron and proton are held together by
electromagnetic force, while planets and celestial objects are held by
gravity. Because of the discretization of
angular momentum postulated in early
quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, and therefore only certain allowed energies.
A more accurate description of the hydrogen atom comes from a purely quantum mechanical treatment that uses the
Dirac equation or even the
path integral formulation to calculate the
probability density of the electron around the proton.
 The most complicated treatments allow for the small effects of
special relativity and
vacuum polarization. In the quantum mechanical treatment, the electron in a ground state hydrogen atom has no angular momentum at all—illustrating how the "planetary orbit" differs from electron motion.
Elemental molecular forms
There exist two different
spin isomers of hydrogen diatomic molecules that differ by the relative
spin of their nuclei.
 In the
orthohydrogen form, the spins of the two protons are parallel and form a triplet state with a molecular spin quantum number of 1 (1⁄2+1⁄2); in the
parahydrogen form the spins are antiparallel and form a singlet with a molecular spin quantum number of 0 (1⁄2–1⁄2). At standard temperature and pressure, hydrogen gas contains about 25% of the para form and 75% of the ortho form, also known as the "normal form".
 The equilibrium ratio of orthohydrogen to parahydrogen depends on temperature, but because the ortho form is an
excited state and has a higher energy than the para form, it is unstable and cannot be purified. At very low temperatures, the equilibrium state is composed almost exclusively of the para form. The liquid and gas phase thermal properties of pure parahydrogen differ significantly from those of the normal form because of differences in rotational heat capacities, as discussed more fully in
spin isomers of hydrogen.
 The ortho/para distinction also occurs in other hydrogen-containing molecules or functional groups, such as water and
methylene, but is of little significance for their thermal properties.
The uncatalyzed interconversion between para and ortho H2 increases with increasing temperature; thus rapidly condensed H2 contains large quantities of the high-energy ortho form that converts to the para form very slowly.
 The ortho/para ratio in condensed H2 is an important consideration in the preparation and storage of
liquid hydrogen: the conversion from ortho to para is
exothermic and produces enough heat to evaporate some of the hydrogen liquid, leading to loss of liquefied material.
Catalysts for the ortho-para interconversion, such as
activated carbon, platinized asbestos, rare earth metals, uranium compounds,
chromic oxide, or some nickel
 compounds, are used during hydrogen cooling.
Covalent and organic compounds
While H2 is not very reactive under standard conditions, it does form compounds with most elements. Hydrogen can form compounds with elements that are more
electronegative, such as
halogens (e.g., F, Cl, Br, I), or
oxygen; in these compounds hydrogen takes on a partial positive charge.
 When bonded to
nitrogen, hydrogen can participate in a form of medium-strength noncovalent bonding with the hydrogen of other similar molecules, a phenomenon called
hydrogen bonding that is critical to the stability of many biological molecules.
 Hydrogen also forms compounds with less electronegative elements, such as
metalloids, where it takes on a partial negative charge. These compounds are often known as
Hydrogen forms a vast array of compounds with
carbon called the
hydrocarbons, and an even vaster array with
heteroatoms that, because of their general association with living things, are called
 The study of their properties is known as
 and their study in the context of living
organisms is known as
 By some definitions, "organic" compounds are only required to contain carbon. However, most of them also contain hydrogen, and because it is the carbon-hydrogen bond which gives this class of compounds most of its particular chemical characteristics, carbon-hydrogen bonds are required in some definitions of the word "organic" in chemistry.
 Millions of
hydrocarbons are known, and they are usually formed by complicated synthetic pathways that seldom involve elementary hydrogen.
Compounds of hydrogen are often called
hydrides, a term that is used fairly loosely. The term "hydride" suggests that the H atom has acquired a negative or anionic character, denoted H−, and is used when hydrogen forms a compound with a more
electropositive element. The existence of the
hydride anion, suggested by
Gilbert N. Lewis in 1916 for group 1 and 2 salt-like hydrides, was demonstrated by Moers in 1920 by the electrolysis of molten
lithium hydride (LiH), producing a
stoichiometry quantity of hydrogen at the anode.
 For hydrides other than group 1 and 2 metals, the term is quite misleading, considering the low electronegativity of hydrogen. An exception in group 2 hydrides is BeH
2, which is polymeric. In
lithium aluminium hydride, the AlH−
4 anion carries hydridic centers firmly attached to the Al(III).
Although hydrides can be formed with almost all main-group elements, the number and combination of possible compounds varies widely; for example, more than 100 binary borane hydrides are known, but only one binary aluminium hydride.
indium hydride has not yet been identified, although larger complexes exist.
inorganic chemistry, hydrides can also serve as
bridging ligands that link two metal centers in a
coordination complex. This function is particularly common in
group 13 elements, especially in
boron hydrides) and
aluminium complexes, as well as in clustered
Protons and acids
Oxidation of hydrogen removes its electron and gives
H+, which contains no electrons and a
nucleus which is usually composed of one proton. That is why H+
is often called a proton. This species is central to discussion of
acids. Under the
Bronsted-Lowry theory, acids are proton donors, while bases are proton acceptors.
A bare proton, H+
, cannot exist in solution or in ionic crystals because of its unstoppable attraction to other atoms or molecules with electrons. Except at the high temperatures associated with plasmas, such protons cannot be removed from the
electron clouds of atoms and molecules, and will remain attached to them. However, the term 'proton' is sometimes used loosely and metaphorically to refer to positively charged or
cationic hydrogen attached to other species in this fashion, and as such is denoted "H+
" without any implication that any single protons exist freely as a species.
To avoid the implication of the naked "solvated proton" in solution, acidic aqueous solutions are sometimes considered to contain a less unlikely fictitious species, termed the "
hydronium ion" (H
). However, even in this case, such solvated hydrogen cations are more realistically conceived as being organized into clusters that form species closer to H
oxonium ions are found when water is in acidic solution with other solvents.
Although exotic on Earth, one of the most common ions in the universe is the H+
3 ion, known as
protonated molecular hydrogen or the trihydrogen cation.
Hydrogen discharge (spectrum) tube
Deuterium discharge (spectrum) tube
Protium, the most common
of hydrogen, has one proton and one electron. Unique among all stable isotopes, it has no neutrons (see
for a discussion of why others do not exist).
Hydrogen has three naturally occurring isotopes, denoted 1
H and 3
H. Other, highly unstable nuclei (4
H to 7
H) have been synthesized in the laboratory but not observed in nature.
H is the most common hydrogen isotope with an abundance of more than 99.98%. Because the
nucleus of this isotope consists of only a single proton, it is given the descriptive but rarely used formal name protium.
H, the other stable hydrogen isotope, is known as
deuterium and contains one proton and one
neutron in the nucleus. All deuterium in the universe is thought to have been produced at the time of the
Big Bang, and has endured since that time. Deuterium is not radioactive, and does not represent a significant toxicity hazard. Water enriched in molecules that include deuterium instead of normal hydrogen is called
heavy water. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for 1
 Heavy water is used as a
neutron moderator and coolant for nuclear reactors. Deuterium is also a potential fuel for commercial
H is known as
tritium and contains one proton and two neutrons in its nucleus. It is radioactive, decaying into
beta decay with a
half-life of 12.32 years.
 It is so radioactive that it can be used in
luminous paint, making it useful in such things as watches. The glass prevents the small amount of radiation from getting out.
 Small amounts of tritium are produced naturally by the interaction of cosmic rays with atmospheric gases; tritium has also been released during
nuclear weapons tests.
 It is used in nuclear fusion reactions,
 as a tracer in
 and in specialized
self-powered lighting devices.
 Tritium has also been used in chemical and biological labeling experiments as a
Hydrogen is the only element that has different names for its isotopes in common use today. During the early study of radioactivity, various heavy radioactive isotopes were given their own names, but such names are no longer used, except for deuterium and tritium. The symbols D and T (instead of 2
H and 3
H) are sometimes used for deuterium and tritium, but the corresponding symbol for protium, P, is already in use for
phosphorus and thus is not available for protium.
 In its
nomenclatural guidelines, the
International Union of Pure and Applied Chemistry (IUPAC) allows any of D, T, 2
H, and 3
H to be used, although 2
H and 3
H are preferred.
muonium (symbol Mu), composed of an
antimuon and an
electron, is also sometimes considered as a light radioisotope of hydrogen, due to the mass difference between the antimuon and the electron.
 Muonium was discovered in 1960.
 During the muon's µs lifetime, muonium can enter into compounds such as muonium chloride (MuCl) or sodium muonide (NaMu), analogous to
hydrogen chloride and
sodium hydride respectively.