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Isotopes of Hydrogen


Ordinary hydrogen exists in three isotopic forms with mass numbers 1, 2 and 3. The isotopes exhibit similar properties (because they all contain one proton and one electron each, i.e. the same electronic configuration) but differ in physical properties (because of the difference in mass number).
In general, hydrogen with mass number 1 (also called protium) undergoes reactions more rapidly than heavy hydrogen (also called deuterium, D). The third isotope with mass number 3 is called tritium (T) and is radioactive.

Physical Properties of Isotopes of Hydrogen


Some physical properties of the three isotopes are given in the table below. The difference in physical properties and reaction rates of the isotopes is made use of in their separation from each other.

Physical Properties of H2, D2 and T2

Property

H2

D2

T2

Molecular mass, m/amu

2.016

4.028

6.016

Melting point, T/K

13.9

18.7

20.6

Boiling point, T/K

20.4

23.7

25.0

Enthalpy of fusion, Hfus/kJ mol-1

0.117

0.197

0.250

Enthalpy of vaporization, Hvap/kJ mol-1

0.904

1.226

1.393

Bond enthalpy, /kJ mol-1 at 298.2 K

435.9

443.4

446.9

Internuclear distance, r/pm

74.14

74.14

(74.14)


Deuterium is invariably prepared by the electrolysis of heavy water, D2O, when deuterium is collected at the cathode. Heavy water itself is manufactured on a large scale by the electrolytic enrichment of normal water. The most important use of heavy water is in the nuclear reactors as a coolant and moderator to slow down the high-energy neutrons. Since it undergoes exchange reactions, heavy water finds use as a tracer to study metabolic processes and chemical reactions. In our country, heavy water is being manufactured in Tuticorin (Tamil Nadu), Baroda (Gujarat), Kota (Rajasthan) and Thal (Maharashtra).




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