Hydrogen is thought to account for approximately \( 75 \% \) of the mass of the universe. Our sun and other stars, for instance, are composed mainly of hydrogen, which serves as their nuclear fuel. On earth, though hydrogen is rarely found in its uncombined state because earth's gravity is too weak to hold such light molecules. Nearly all the \( \mathrm{H}_{2} \) 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, fats and literally millions of other compounds and is ninth most abundant element on a mass basis.
There are three isotopes of hydrogen : protium or ordinary hydrogen \( \left({ }_{1}^{1} \mathrm{H}\right) \); deuterium or heavy hydrogen \( \left({ }_{1}^{2} \mathrm{H}\right. \) or D) and tritium ( \( { }_{1}^{3} \mathrm{H} \) or T). The chemical properties of the three isotopes
are similar because they have same electronic configuration. The physical properties of three isotopes are different due to difference in their masses, i.e., isotope effects.
The reaction of 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.
\[
2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(g) \quad \Delta H^{\circ}=-484 \mathrm{~kJ}
\]
Hydrogen is an enormously attractive fuel because it is environmentally clean. 'Hydrogen economy' is a new field in which it is thought that our energy needs can be meet by gaseous, liquid and solid hydrogen. Since hydrogen is not a naturally occurring substance like coal, oil or natural gas, energy must be expended to produce hydrogen before it can be used. Current research, therefore, focuses on finding cheaper methods for extracting hydrogen.
(A) Oxygen
(B) Nitrogen
(C) Hydrogen
(D) Helium
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