Hydrogen accounts for approximately `75%` of the mass of the universe.
Channel: Doubtnut
Date: 26/07/2020

Hydrogen accounts for approximately `75%` of the mass of the universe. Hydrogen serves as the nuclear fuel of our Sun and other stars, and these are mainly composed of hydrogen. On the earth, though hydrogen is rarely found in the uncombined state. Since the earth's gravity is too weak to hold such light molecules, nearly all the `H_2` originally present in the earth's atmosphere has been lost to space. In the earth's crust and oceans, hydrogen is found in water, petroleum, proteins, carbohydrates and other compounds and it is the ninth most abundant element on a mass basis. Hydrogen has three isotopes : hydrogen or protium `()`, deuterium or heavy hydrogen (D or ), tritium `(T or )`. The physical properties of the three isotopes are different due to the difference in their masses, i.e. isotope effect. The chemical properties of the three isotopes are similar as they have the same electronic configuration. Reaction between hydrogen and oxygen is highly exothermic, and gas mixtures that contain as little as `4%` by volume hydrogen in oxygen (or in air) are highly flammable and potentially explosive.  `2H_(2(g))+O_(2(g)), DeltaH^(ɵ)=-485 kJmol^(-1)`  As hydrogen is environmentally clean it is an enormously attractive fuel. 'Hydrogen economy' is an emerging field in which it is thought that our energy needs can be met by gaseous, liquid and solid hydrogen. As hydrogen is no a naturally occuring substance such as coal, oil or natural gas, energy must be exploaded to produce hydrogen before it can be used.  Hydrogen, `H_2` is very less abundant in the atmosphere due to