Hydrogen, (Chemical Element, Symbol H, Atomic Number 1)




Hydrogen is the first element of the Periodic Table because it is has the smallest atom of all the elements, consisting of just one proton, one neutron and one electron. It is also believed to be the most abundant element in the universe. (That is, at the moment, scientists think that there are more atoms of Hydrogen in the universe than there are atoms of any other element.)

Hydrogen gas (H2) was first produced artificially in the early 1500s as a result of reacting metals with strong acids but it was not realised at that time that the resulting gas was a previously undocumented element. Hydrogen gas was finally recognised as a discrete substance as a result of Henry Cavendish's work in the 1760-80s, and was named 'hydrogen' in 1783. It was first liquefied in 1898 as a result of the inventions of regenerative cooling and the vacuum flask. Solid hydrogen was formed for the first time in 1899. The isotopes of hydrogen deuterium and tritium were discovered in the 1930s.

The element Hydrogen at standard temperature and pressure ('standard conditions')

Hydrogen is a colourless, odourless, tasteless gas at room temperature and pressure.

Hydrogen gas consists of molecules, rather than atoms, of hydrogen. Two atoms of hydrogen form each hydrogen molecule, hence molecules of hydrogen are said to be diatomic and the chemical symbol for hydrogen gas is H2.

Hydrogen Atom

The hydrogen atom is the simplest of all atoms because is consists of only one proton at its centre and one electron (forming the corresponding negative charge).

Size of an atom of hydrogen:
The diameter of a single hydrogen atom (in its ground state) is 10-10 meters. There are several different ways to write this number. Writing it as 10-10m is useful because it is a short way to express this distance and because it gives an immediate indication of the scale via the suffix "-10". If this style of writing numbers is unfamilar, see information about scientific numbers.

Compounds of Hydrogen

The element hydrogen forms many different compounds by linking with atoms of other elements (several atoms linked together form molecules). Important compounds of hydrogen include water (H2O) and many different hydrocarbons - that is compounds that include the elements carbon and hydrogen and are described by the subject of organic chemistry. Other compounds of hydrogen include many '-hydrides', e.g. calcium hydride (which is used as a desiccant). Hydrogen also plays an important role in acids.

Hydrogen Bonding

About chemical bonds in general: When chemical elements connect to each other to form molecules, chemical 'bonds' are said to be formed between the atoms that are attached to each other. There are different types of chemical bonds, including covalent bonds, ionic bonds, van der Waals forces (or 'interactions') and hydrogen bonds. This is mentioned here because hydrogen bonds are the only ones named after a specific element. Hydrogen bonds are so strong that they can have significant effects on the physical properties of the substances involved. The most common example is water, whose boiling point is unexpectedly high due to the effects of Hydrogen bonding.

Isotopes of Hydrogen

Hydrogen has several isotopes and is extremely unusual because its isotopes are referred to by different names, not just as hydrogen-[mass number of the isotope]. The most common hydrogen isotope by far is hydrogen-1. That is, a form of the element hydrogen whose nucleus contains a single proton only. This isotope is generally assumed, hence known known simply as 'hydrogen', however is also has the name protium which maybe be used in the context of information about isotopes of hydrogen.

Other isotopes of hydrogen include:


Name of Isotope:

Content of Nucleus:




1 proton

Most common isotope, 99.98% known hydrogen. Stable, naturally occuring.



1 proton,
1 neutron

Stable (not radioactive), naturally occurring, but rare. Water enriched in molecules that include deuterium instead of protium is called 'heavy water' and may be used as a neutron moderator and coolant for nuclear reactor, also as considered a possible fuel for nuclear fusion.



1 proton,
2 neutron

Naturally occurring but very rare and radioactive. Tritium decays into helium-3 through β− decay with a half-life of 12.32 years. Tritium has has various uses, including in thermonuclear fusion weapons, as a tracer in isotope geochemistry, in special in self-powered lighting devices, and (formerly) in chemical and biological labeling experiments as a radiolabel.



1 proton,
3 neutron

Highly unstable, radioactive, not known to occur naturally but has been synthesised in laboratories and its half-life measured.



1 proton,
4 neutron

Highly unstable, radioactive, not known to occur naturally but has been synthesised in laboratories and its half-life measured.



1 proton,
5 neutron

Highly unstable, radioactive, not known to occur naturally but has been synthesised in laboratories and its half-life measured.



1 proton,
6 neutron

Highly unstable, radioactive, not naturally occurring. First synthesised in 2003.

Uses of Hydrogen (Gas)

The element hydrogen occurs naturally as a diatomic gas (H2) known as 'hydrogen gas'.

Uses of elemental hydrogen include:

  • Historically - Airships: In the early 20th Century, esp. 1930s, huge airships transporting passengers and cargo over long distances used hydrogen gas because hydrogen is the lightest (least dense) gas. However, as well as being light, hydrogen gas is also highly flammable. After the "Hindenburg Disaster" in which the luxury airship the Hindenburg burst into flames on encountering an electrical spark when landing in New Jersey (USA), hydrogen gas ceased to be used for airships to be replaced by the much heavier (though still relatively light) helium gas.
  • Weather Balloons: Meteorologists still use hydrogen gas to fill weather balloons, which are very much smaller than airships - hence hydrogen is generally considered safe enough. These balloons are about 2 m in diameter and float up into the atmosphere carrying a instruments that record details of atmospheric conditions.
  • Rocket Fuel: The highly inflammable nature of hydrogen gas, especially when mixed with pure oxygen, lead to its use as rocket fuel. Energy for 'lift-off' can be generated by combining liquid hydrogen with liquid oxygen. However, even this use of hydrogen can be dangerous, e.g. the 1986 U.S. Space Shuttle Challenger disaster when a flame accidentally ignited liquid hydrogen stored in a fuel tank.
  • Hydrogen Fuel Cells for Electricity: Hydrogen fuel cells (batteries) can be used as a source of electricty but and are very efficient, but currently also highly expensive to produce.
  • Hydrogen Fuel Cells for Transport: There is a small but increasing number of hydrogen-fueled vehicles in service in the USA, Australia and elsewhere. They store hydrogen (in either gas or liquid form) and convert it into electricity by means of a fuel cell in order to drive the motor of the vehicle. However, the current cost of fuel cell vehicles is much more than that of conventional petrol / diesel engine vehicles, largely due to the high cost of production of fuel cells.
  • Thermonuclear Fusion: When heated to extreme temperatures the nuclei of hydrogen atoms will fuse to create helium nuclei. This fusion results in the release of an enormous amount of energy, called thermonuclear energy. It is hoped that scientists will eventually find a way to produce and harness such 'clean' energy for peaceful applications, such as generation of electricity for homes and businesses.

The element hydrogen also exists in combination with various other elements in many different compounds, a common example being water (H2O). The uses and importance of compounds that include hydrogen are too numerous to list. The following are examples of cases in which the presence or action of hydrogen is particularly important:

  • Production of Margarine: Hydrogen is used in the food industry to 'hydrogenate' oils or fats, enabling margarine to be produced from vegetable oils.
  • Extracting Metals: Hydrogen is used as reducing agent for extracting metal from metal ores, e.g. tungsten oxide is mined then heated in the presence of hydrogen - resulting in oxygen being released from tungsten oxide to combine with the hydrogen to form water and the metal tungsten.
  • Refining Crude Oil: Hydrogen is used in petrochemical plants for processes such as hydrodealkylation, hydrodesulfurisation, and hydrocracking.
  • Manufacture of Fertilizers: Hydrogen and Nitrogen gases are combined using the 'Haber Process' to form the compound ammonia (NH3), which has several uses, including as a fertilizer to feed agricultural crops e.g. in the USA.
  • Production of other industrial chemicals: e.g. hydrogen is used to produce methanol and hydrochloric acid.

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