1 HYDROGEN H (Greek: hydro genes = water forming)
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.
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.