Hydrogen is the lightest atom with only one electron. Loss of this electron results in an elementary particle, the proton. Thus, it is unique in character.
It has three isotopes, namely : protium (1 1 H), deuterium (D or 2 1H) and tritium (T or3 1H). Amongst these three, only tritium is radioactive. Inspite of its resemblance both with alkali metals and halogens, it occupies a separate position in the periodic table because of its unique properties.
Hydrogen is the most abundant element in the universe. In the free state it is almost not found in the earth’s atmosphere.
However, in the combined state, it is the third most abundant element on the earth’s surface. Dihydrogen on the industrial scale is prepared by the water-gas shift reaction from petrochemicals. It is obtained as a byproduct by the electrolysis of brine.
The H–H bond dissociation enthalpy of dihydrogen (435.88 kJ mol –1 ) is the highest for a single bond between two atoms of any elements.
This property is made use of in the atomic hydrogen torch which generates a temperature of ~4000K and is ideal for welding of high melting metals.
Though dihydrogen is rather inactive at room temperature because of very high negative dissociation enthalpy, it combines with almost all the elements under appropriate conditions to form hydrides.
All the type of hydrides can be classified into three categories: ionic or saline hydrides, covalent or molecular hydrides and metallic or non-stoichiometric hydrides.
Alkali metal hydrides are good reagents for preparing other hydride compounds. Molecular hydrides (e.g., B2H6, CH4, NH3, H2O) are of great importance in day-to-day life.
Metallic hydrides are useful for ultrapurification of dihydrogen and as dihydrogen storage media. Among the other chemical reactions of dihydrogen, reducing reactions leading to the formation hydrogen halides, water, ammonia, methanol, vanaspati ghee, etc. are of great importance.
In metallurgical process, it is used to reduce metal oxides. In space programmes, it is used as a rocket fuel. In fact, it has promising potential for use as a non-polluting fuel of the near future (Hydrogen Economy). Water is the most common and abundantly available substance.
It is of a great chemical and biological significance. The ease with which water is transformed from liquid to solid and to gaseous state allows it to play a vital role in the biosphere. The water molecule is highly polar in nature due to its bent structure.
This property leads to hydrogen bonding which is the maximum in ice and least in water vapour.
The polar nature of water makes it:
(a) a very good solvent for ionic and partially ionic compounds;
(b) to act as an amphoteric (acid as well as base) substance; and
(c) to form hydrates of different types.
Its property to dissolve many salts, particularly in large quantity, makes it hard and hazardous for industrial use.
Both temporary and permanent hardness can be removed by the use of zeolites, and synthetic ion-exchangers.
Heavy water, D2O is another important compound which is manufactured by the electrolytic enrichment of normal water. It is essentially used as a moderator in nuclear reactors.
Hydrogen peroxide, H2O2 has an interesting non-polar structure and is widely used as an industrial bleach and in pharmaceutical and pollution control treatment of industrial and domestic effluents.