A colourless, odourless diatomic gas of valency 1. The least dense (lightest) and most abundant element in the Universe, forming diatomic molecules, H₂, and when burnt water, H₂O. Has both metallic and non-metallic properties. Mixed with air, it is dangerously explosive on ignition. Forms a blue metallic solid below -259.14ºC. Boils at -252.7ºC. It is widely distributed as water, occurs in many minerals, eg petroleum, and in living matter.

Chemically, hydrogen is ambivalent; it has one electron more than a closed shell so should behave as an alkali metal and belong to group 1, but it also has one electron fewer than a closed shell and so should belong to the halogens of group 17. However, it stubbornly refuses to conduct electricity like an alkali metal even when pressurised to 2 million atmospheres (200 GPa), nearly 10 times the 25 GPa calculated as necessary, but does succumb under enormous impact when the pressure has risen to 140 GPa and the temperature to 3000 Kelvin, the liquid hydrogen then conducts like an alkali metal, albeit at 10 times the density at which alkali metals conduct electricity. Unlike alkali metals, which exist as ions in a regular lattice which conduct electricity, metallic hydrogen remains strictly covalently bonded H₂ molecules. The planet Jupiter is thought to consist of a liquid outer layer of non-metallic molecular hydrogen, with a liquid monatomic metallic hydrogen core.

Hydrogen is manufactured chiefly as a by-product of electrolysing caustic soda, water gas and gas cracking. It is used in the Haber ammonia making process for the fixation of nitrogen; in the hardening of fats (eg in the manufacture of margarine) and hydrogenation of oils; in the manufacture of hydrochloric acid; for filling small lighter-than-air balloons (and much bigger ones until the Hindenberg airship disaster); as a reducing agent for organic synthesis; in metal refining; and in oxy-hydrogen and atomic hydrogen welding torches. With liquid oxygen as oxidiser, liquid hydrogen is used as the propellant in some rocket engines. Hydrogen dissolves readily in many transition elements, which could be used as the basis for a safe hydrogen fuel storage in vehicles. Unlike oxygen, hydrogen is present in all acids.

Diatomic hydrogen can exist in two spin states: the lower energy para-hydrogen with paired (anti-parallel) nuclear spins, and three higher energy states with parallel nuclear spins collectively called ortho-hydrogen. At normal temperatures, they exist in the ratio 1:3, respectively. The two forms have slightly differing properties, and the very small thermal coupling between the two spin alignments prevents hydrogen being used for cryogenic cooling. Hydrogen solidifies below 14.5 Kelvin crystallizing in the hexagonal system. Below 4.5 Kelvin it changes into a cubic structure. Near 1.3 Kelvin, it becomes a mixture of hexagonal close packed and face centred cubic depending upon the ratio of ortho/para hydrogen.

Three oxides are known: water, H₂O, (H-O-H); hydrogen peroxide, H₂O₂, an oxidiser and bleach, (H-O-O-H). The third, hydrogen trioxide, H₂O₃, (H-O-O-O-H) was discovered in 1994 by oxidizing 2-ethyl anthraquinone with ozone, and was then found to be normally present in the atmosphere all along. It is relatively stable at room temperature, more so than hydrogen peroxide.

WATER (and ice) Water, H2O, is one of the strangest substances known with quite unexpected properties. An analogous compound of water, hydrogen sulphide, H2S, remains gaseous even well below 0ºC. But remarkably the lighter hydrogen oxide (which should have the even lower boiling point of -80ºC) actually boils at 100ºC, an anomaly of 180ºC. Water also has an anomalously high thermal capacity (specific heat) and a strange reluctance to being compressed. Water should also freeze at -90ºC but instead freezes at 0ºC. These anomalies are due entirely to the (very weak) hydrogen-bonds between the hydrogen atoms of one molecule and the oxygen atoms of another molecule of H2O, which keep making and breaking. Water is essentially polymeric, which increases the effective mass of a molecule of water lowering its boiling point (and melting point). Water continuously forms and breaks hydrogen bonds with its neighbouring molecules forming polymers. Apart from the dimer, they are planar rings up to and including the pentamer, but the hexamer is three dimensional with a closed-cage and is the more stable. Water is more like a liquid crystal, containing polymers with up to 200 water molecules at zero Celsius, and even at boiling point not all the hydrogen bonds are broken. The hydrogen bonds are the result of zero-point energy fluctuations, a quantum phenomenon. But replace the hydrogen atoms in water with hydrogens next heavier isotope, deuterium, and the hydrogen bonds vanish, and the resulting 'heavy water' is poisonous to all living things except some primitive organisms! It seems that water is essential to the correct folding of proteins in cells, orchestrated by the swarming clusters of water molecules, a characteristic imparted by their hydrogen bonds. Water is also essential to the working of DNA. Without surrounding water molecules, DNA can't react properly with proteins. It is not just a case of water being used a a solvent, but it actively participates in and enables DNA processes. It seems the water molecules can relay, by electrostatic forces, the state of DNA molecules to an approaching protein before it gets near the DNA, and enticing it nearer or forcing it away if it is not quite right. Even as ice, the water molecules are not fixed permanently in position, but continually jostle about into empty niches. Ice is a supercooled liquid. It is suspected that in water there are two different kinds of hydrogen bond depending upon temperature, one weaker than the other, with the strong variety changing into the weak variety above 37ºC. It may be no coincidence that this is the temperature of the human body. Water freezes at 0ºC but strangely has a maximum density (of exactly 1) at 3.98ºC. This means that water, unlike other liquids, expands as it is cooled from 3.98ºC to 0ºC. Moreover, the density of ice at 0ºC is 0.9168 whereas that of water at 0ºC is 0.99987. That is to say, water increases in volume by 9% as it freezes, and this is what bursts 'frozen' water pipes (they only leak when the water melts again). (The density of water at 100ºC is 0.958). Water also has an anomalously high surface tension, a very high dielectric constant, an exceedingly high specific heat, a very high latent heat of fusion, and the highest latent heat of vaporisation of any known substance. It is also able to dissolve a diverse range of substances. Water behaves as if it was partly H+ + OH-, being a protonic acid and a hydroxyl alkali at the same time, and has a neutral pH of exactly 7.0, being neither acidic nor alkaline. Water is partially polar with a 40% ionic nature, which is responsible for its high dielectric constant, and favours dissolving polar substances, like sodium chloride, which it divides into positively charged sodium ions and negatively charged chlorine ions. Water boils at 100C (at NTP) when it becomes steam. Steam can be further heated and pressurised until, at the critcal point, reached when the temperature reaches 374C and the pressure 22.064 Mega Pascals, it becomes denser than the water which it once was. Further heating or pressurisation results in the steam becoming 'super-critical' (above the critical point) where liquid-like clusters of water molecules float in a vapour-like phase. In this phase it behaves quite differently, a fourth state of matter. Super critical steam is now used to drive steam turbines for electrical power generation, resulting in an increase in efficiency.

ICE (and water) Moreover, the peculiarities of H2O do not stop when it becomes ice. The density of the solid form, ice (density 0.92) is, most unusually, lower than the density of its liquid form, water; ice floats on water! The oxygen atoms in ice occupy tetrahedral sites with the same lattice structure as diamond where there are but four nearest neighbours instead of twelve as in the hexagonal close packed structure. The diamond structure is relatively empty. The hydrogen atoms in this lattice occupy sites between the oxygen atoms and continually oscillate two and fro between two neighbouring oxygen atoms, being first H·OH then HO·H. But in water, there are, on average, more than four nearest neigbours, water is denser that its ice. Water is not a linear molecule, but is bent at an angle of 105º, but in ice it deforms further to the tetrahedral angle of 109º and this accounts for the diamond structure and the increase in volume on freezing. Bulk ice at normal pressures does not crystallize but is an amorphous solid akin to a supercooled liquid; a glass. It flows very slowly. Increased pressure will cause localised melting, which is why ice is so slippy and skaters can skate upon it. The thin layer of water that forms between ice and a skaters blade has been found to have an extraordinarily high density, 1.17, and is still liquid even at -17ºC, but little else is known about it although it is being actively studied. At a pressure of about 2400 atmospheres, water remains liquid at temperatures of -20ºC. Ice will crystallize at increased pressures, in fact, ice has a greater number of crystalline structures (polymorphs) than any other known solid, more than 10 in all, known by Roman numerals ice I to ice X. Of these, ice VIII has the highest density (1.5) and is found on Ganymede under the extreme pressure. The melting points of some of these high pressure forms is higher than 0ºC. Ice (and snow) will slowly evaporate below 0ºC without melting, a process called ablation. Without these unique properties, water could not support life. The high specific heat of water enables the oceans to store and transport vast amounts of heat energy from equatorial regions of the globe to the polar regions, thus driving the weather.

Weak hydrogen-bonds also occur between fluorine and hydrogen atoms; between nitrogen and hydrogen atoms; as well as between oxygen and hydrogen atoms, with similar consequences to boiling points. Thus HF has a much higher boiling point than the heavier HCl, and so too does NH₃ in comparison to PH₃.

Heavy water, D₂O, is not a substitute for water. Given only heavy water to drink, frogs will die of thirst.

In the form of water, hydrogen is of great importance in the moderation (slowing down) of neutrons, because hydrogen atoms are the only ones of similar mass to a neutron and are therefore capable of absorbing an appreciable proportion of the energy of a neutron on collision. Light water, with the lightest isotope of hydrogen, protium, is the best moderator, but also has an unwanted high affinity for absorbing rather than scattering the neutrons, forming deuterium. Heavy water, with deuterium, has a very much smaller probability of absorbing neutrons, but when it does so, forms the heavier and dangerously radioactive isotope, tritium.

Many compounds with nitrogen are known, ammonia, NH₃ and its isomorphous analogue hydrazoic acid, HN₃; hydrazine, N₂H₄; hydrazine azide, N₂H₄.HN₃; and ammonium azide, NH₄N₃. Amides and amines are compounds where one (amino, NH₂), two (imino, NH) or three (nitrile, N) of the hydrogen atoms in ammonia have been replaced by some other group(s). See nitrogen.

Hydrogen has three isotopes: hydrogen-1 or protium, hydrogen-2 or deuterium, and the radioactive hydrogen-3 or tritium; containing 0, 1, and 2 neutrons, respectively. Deuterium or heavy hydrogen is present in hydrogen at 0.015% concentration, and when combined with oxygen, is known as heavy water, normally present as both HDO and D₂O in ordinary water. The purer D₂O is used as a moderator of neutrons in nuclear reactors. Heavy water can be extracted from water by either fractional distillation (because it has a slightly higher boiling point (101.42ºC)) or by electrolysis where there is a slight preference for evolution of H₂ rather than HD or D₂ at one electrode.

On Venus, which is much nearer the Sun than Earth, the hydrogen-1 isotope in water has preferentially evaporated from Venus's atmosphere, leaving Venus now with little water and a hydrogen-1 to hydrogen-2 isotopic ratio which is 150 times that on Earth. It seems what little water is now left on Venus is mostly heavy water.

Tritium, with a half-life of 12.3 years, emits beta radiation and has been used contained in phials together with a phosphor, in luminescent telephone dials (until banned because of the damaging radiation it emits)! Tritium is produced naturally by cosmic ray bombardment of the upper atmosphere. Tritium is also generated in small amounts in nuclear power stations as a by product of the fission reactions. Deuterium and tritium repel each other less than any other isotopes so are used in fusion research and as the nuclear fuel in thermonuclear H-bombs. The nuclear fusion of deuterium with tritium is highly exothermic, emitting the energy as gamma rays, ²H + ³H = ⁴He + n + . So not surprisingly, substantial quantities of tritium are purposely made by bombarding lithium-6 with neutrons produced within a nuclear reactor (⁶Li + n = ³H + ⁴He). The tritium is then reacted chemically with lithium-6 to form lithium tritide, ⁶LiT, which, together with rather more lithium deuteride, ⁶LiD, is used in modern thermonuclear hydrogen bombs, where, during the explosion, the lithium-6 is bombarded with neutrons creating more tritium. Fortuitously, the nuclear fuel within H-bombs requires replenishing every 10 years or so because the tritium decays radioactively. But by using lithium-6, which isn't radioactive, to generate more tritium at detonation time, the 'use before date' can, unfortunately, be forestalled a little.

Hyper hydrogen-4, is a tritium nucleus with a lambda particle (a hyper particle with one up, one down and one strange quark) in the nucleus, and decays in 10^-10 secs. Such nuclei are known as hypernuclei.

In interstellar space, the molecular ion H₃⁺, where the three hydrogen atoms are arranged in a triangle and share the two electrons equally, readily forms wherever hydrogen gas is ionized by radiation to H₂⁺. The H₂⁺ reacts with H₂ producing H₃+ which is the more stable. The un-ionized molecule H₃ cannot exist. In the ultra-high vacuum of interstellar space where atomic collisions are exceedingly rare, it is the electrostatic attractive force between ionized molecules which enormously increases the probability of collisions and hence chemical reactions, producing some highly un-expected molecules like the cyanopolyynes, HC₃N, HC₅N, HC₇N, HC₉N and HC₁₁N which are higher analogues of hydrocyanic acid, HCN.

Anti-hydrogen, the first element of the anti-periodic table containing anti-matter, was made in 1995 by colliding a beam of xenon atoms with a beam of anti-protons in an accelerator. Whereas hydrogen consists of an electron bound to a central proton, anti-hydrogen consists of a positron (or anti-electron) bound to a central anti-proton. Nine anti-atoms were made, but these were annihilated by collisions with ordinary matter within 30 nano seconds of creation. A matter/antimatter reactor, the most efficient energy conversion process known, is still a long way off.

Claim to fame: Hydrogen has the smallest atom, atomic radius 37pm, but is the least dense element, density 0.084 grams per litre at 1013.25mb pressure and is the most abundant element in the Universe.