hydrogen diffusion flame

Hydrogen diffusion flame in the Natural Gas Combustion Apparatus. Credit: National Energy Technology Laboratory.

hydrogen discharge tube

Hydrogen discharge tube. Image copyright: smart-elements.com.

Hydrogen is a colorless, highly flammable, gaseous element. It is the first and lightest element in the periodic table. Hydrogen was first recognized as a distinct substance by Henry Cavendish in 1776, although it had been made much earlier by chemists and alchemists in their experiments with metals and acids. It was named by Antoine Lavoisier from the Greek hydro ('water') and genes ('forming').


Hydrogen is the most abundant element in the universe (see elements, cosmic abundance) and one of the most important to life as we know it.


Hydrogen atoms are the least massive of all atoms. The lightest and most common isotope of hydrogen, 1H, is known as protium and consists of one proton and one electron. Two other isotopes of hydrogen exist. They are deuterium, or "heavy hydrogen", with a nucleus containing one neutron and one proton, and radioactive tritium with a nucleus containing two neutrons and one proton. Hydrogen is the only element whose isotopes have been given different names.


atomic number 1
relative atomic mass 1.0079
electron configuration 1s2
atomic radius 78 pm
relative density 0.08988
melting point -259.14°C (-434.45°F)
boiling point -252.87°C (-455.17°F)
oxidation states 1, -1


Chemistry of hydrogen

Hydrogen is fairly reactive, giving hydrides with most other elements on heating, and a moderate reducing agent. It belongs in no definite group in the periodic table, but has some resemblance to the halogens in forming the ion H-, and to the alkali metals in forming the ion H+ (see acid); it is always monovalent. See also hydrogen bond.


Uses of hydrogen

Large amounts of hydrogen are used commercially for making ammonia by the Haber process, and for the hydrogenation of fats and oils. It is also used in methanol production, in hydrodealkylation, hydrocracking, and hydrodesulfurization, and in oxyhydrogen torches. Liquid hydrogen is employed as a rocket fuel.


The hydrogen fuel cell is a developing technology that will allow much electrical power to be obtained using a hydrogen gas.


Preparation of hydrogen

Hydrogen can be prepared, industrially and in the laboratory, by a variety of methods, including:


· steam on heated carbon
· decomposition of certain hydrocarbons with heat
· reaction of sodium or potassium hydroxide on aluminum
· electrolysis of water
· displacement from acids by certain metals.


Ortho- and para- forms of hydrogen

Quite apart from isotopes, it has been shown that under ordinary conditions hydrogen gas is a mixture of two kinds of molecules, known as orthohydrogen and parahydrogen, which differ from one another by the spins of the protons in their nuclei. In orthohydrogen the protons have opposite spin; in parahydrogen they have the same spin. Normal hydrogen at room temperature contains 25% of the para- form and 75% of the ortho- form. The ortho- form cannot be prepared in the pure state. Since the two forms differ in energy, the physical properties also differ. The melting and boiling points of parahydrogen are about 0.1°C lower than those of normal hydrogen.


Hydrogen in space

Hydrogen makes up the bulk of most stars and gas giant planets. In space, it may exist in atomic (see H I region), ionized (see H II region), or molecular form (see molecular cloud). The presence of atomic hydrogen in space can be mapped by observations of the 21-centimeter line.