Isotopes are defined as the elements which have the different mass number but the same atomic number. The three isotopes of hydrogen are Protium \(\left( {{}_1^1{\rm{H}}} \right),\) Deuterium \(\left( {{}_1^2{\rm{H}}\,{\rm{or}}\,{\rm{D}}} \right)\) and Tritium \(\left( {{}_1^3{\rm{H}}\,{\rm{or}}\,{\rm{T}}} \right).\) These isotopes are different from each other owing to the different number of neutrons present in them.

Protium has only one proton but no neutron and one electron is revolving around the nucleus in its only shell. Deuterium has one proton, one neutron, and one electron. Tritium has two neutrons and one proton in its nucleus. In this article, we will discuss about isotopes of hydrogen in detail.

What is an Isotope?

Isotopes are the different atoms of the same element which have the same atomic number but different mass numbers. Isotopes are also defined as atoms with the same number of protons and a different number of neutrons.

The three isotopes of hydrogen are protium, deuterium, and tritium.

How to Represent Isotopes of Hydrogen?

The isotopes of an element are usually represented by putting the atomic number as the subscript and their mass number as the superscript on the symbol of the element. Thus, the three isotopes of hydrogen are represented as follows:

1. Protium: \({{}_1^1{\rm{H}}}\)
2. Deuterium: \({{}_1^2{\rm{H}}\,{\rm{or}}\,{\rm{D}}}\)
3. Tritium: \({{}_1^3{\rm{H}}\,{\rm{or}}\,{\rm{T}}}\)

Percentage of Each of the Isotopes of Hydrogen

The most abundant isotope of hydrogen is protium. This occurs in natural hydrogen to the extent of \(99.9844\% \) the remaining \(0.0156\% \) being deuterium mostly in the form of HD. Tritium is unstable because of its radioactive nature, occurs only in traces.

The natural abundance of \({}_1^1{\rm{H}}:{\mkern 1mu} {}_1^2{\rm{H}}:{\mkern 1mu} {}_1^3{\rm{H}} = 1:{\mkern 1mu} 1.56 \times {10^{ – 2}}:{\mkern 1mu} 1 \times {10^{ – 18}}\)

Protium or Ordinary Hydrogen \(\left( {{}_1^1{\rm{H}}} \right):\)

This is the most abundant isotope of hydrogen. Since its atomic number is \(1\) and mass number is also \(1.\) Its nucleus has only one proton but no neutron and one electron is revolving around the nucleus in its only shell, i.e., K shell.

Deuterium or Heavy Hydrogen \(\left( {{}_1^2{\rm{H}}\,{\rm{or}}\,{\rm{D}}} \right):\)

Its natural abundance is \(0.0156\% .\) It is usually prepared by the electrolysis of heavy water \(\left( {{{\rm{D}}_{\rm{2}}}{\rm{O}}} \right)\)Since its atomic number is \(1\) and its mass number is \(2,\) its nucleus has one proton and one neutron while one electron is present in the K shell. Therefore, it is generally denoted by the symbol D.

Deuterium can be obtained from the fractional distillation of liquid hydrogen. 

Which is the Radioactive Isotope of Hydrogen?

Tritium \(\left( {{}_1^3{\rm{H}}\,{\rm{or}}\,{\rm{T}}} \right)\) is the least abundant, i.e., \({10^{ – 16}}\% \) of all the isotopes of hydrogen and is formed in the upper atmosphere by reactions induced by cosmic rays. It is radioactive with a short half-life of \(12.33\) years. It decays by \({\rm{\beta }}\)-emission with no \({\rm{\alpha }}\)-radiations.

\({}_1^3{\rm{H}} \to {}_2^3{\rm{H}} + {}_{ – 1}^0{\rm{e}}\)

It is prepared artificially by the bombardment of nitrogen or an isotope of lithium with neutrons:

\({}_7^{14}{\rm{N}} + {}_0^1{\rm{n}} \to {}_6^{12}{\rm{C}} + {}_1^3{\rm{H}}\)
\({}_3^6{\rm{Li}} + {}_0^1{\rm{n}} \to {}_2^4{\rm{He}} + {}_1^3{\rm{H}}\)

Since its mass is \(3\) and atomic number is \(1,\) its nucleus has one proton and \(2\) neutrons while one electron is present in K-shell. It is usually represented by the symbol T.

Uses of Tritium

Tritium is used to make thermonuclear devices and for researching fusion reactions as a means of producing energy. Tritium gas is usually stored by making \({\rm{U}}{{\rm{T}}_3},\) which on heating to \(673\,{\rm{ K}}\) releases \({{\rm{T}}_{\rm{2}}}\).

\(2{\rm{U}}{{\rm{T}}_3}\, \to \,2{\rm{U}}\, + \,3{{\rm{T}}_2}\)

It is widely used as a radioactive tracer since it is relatively cheap and easy to work with.

Atomic and Physical Properties of Isotopes of Hydrogen

The atomic and physical properties of isotopes of hydrogen are as follows:

Property	Protium	Deuterium	Tritium
Relative abundance	\(99.985\% \)	\(0.00156\% \)	\({10^{ – 18}}\)
Relative atomic mass \(\left( {{\rm{g}}/{\rm{mol}}} \right)\)	\(1.008\)	\(2.014\)	\(3.016\)
Density \(\left( {{\rm{g}}/{\rm{L}}} \right)\)	\(0.09\)	\(0.18\)	\(0.27\)
Melting point \(\left( {\rm{K}} \right)\)	\(13.93\)	\(18.73\)	\(20.62\)
Boiling point \(\left( {\rm{K}} \right)\)	\(20.39\)	\(23.67\)	\(25.0\)
Enthalpy of fusion \(\left( {{\rm{kJ}}/{\rm{mol}}} \right)\)	\(0.117\)	\(0.197\)	—
Enthalpy of vaporisation \(\left( {{\rm{kJ}}/{\rm{mol}}} \right)\)	\(0.904\)	\(1.226\)	—-
Bond dissociation enthalpy \(\left( {{\rm{kJ}}/{\rm{mol}}} \right)\) at \(298.2\, {\rm{K}}\)	\(435.88\)	\(443.35\)	—-
Internuclear Distance \(\left( {{\rm{pm}}} \right)\)	\(74.14\)	\(74.17\)	—
Ionization energy \(\left( {{\rm{kJ}}/{\rm{mol}}} \right)\)	\(1312\)	—	—
Electron gain enthalpy \(\left( {{\rm{kJ}}/{\rm{mol}}} \right)\)	\(-73\)	—	—-
Covalent radius \(\left( {{\rm{pm}}} \right)\)	\(37\)	—	—
Ionic radius \(\left( {{{\rm{H}}^ – }} \right)\left( {{\rm{pm}}} \right)\)	\(208\)	—	—
Nuclear spin quantum number	\(1/2\)	\(1\)	\(1/2\)
Radioactive stability	Non-radioactive, stable	Non-radioactive,  Stable	Radioactive emits low energy \({\rm{\beta }}\) -particles, \({{\rm{t}}_{1/2}} = 12.33\, {\rm{y}}\)

Electronic Structure of Isotopes of Hydrogen

All the 3 isotopes of hydrogen, i.e., protium, deuterium, and tritium, have the same atomic number \(\left( {{\rm{Z}} = 1} \right);\) therefore, they have only one electron in the K-shell and one proton in the nucleus. However, since the mass number of the three isotopes is different, they differ from one another in the number of neutrons in the nucleus.

Calculation of the number of neutrons:

Atomic number = Number of protons = number of electrons in the neutral atom

Atomic mass = Number of protons + Number of neutrons

Number of neutrons = Atomic mass – Number of protons

Therefore, the Number of neutrons in protium, \({}_1^1{\rm{H}} = 1 – 1 = 0\)
Number of neutrons in deuterium, \({}_1^2{\rm{H}} = 2 – 1 = 1\)
Number of neutrons in tritium, \({}_1^3{\rm{H}} = 3 – 1 = 2\)
What is the number of the proton, electron, and neutron in \({}_1^1{\rm{H}},\;\;{}_1^2{\rm{H}}\) and \({}_1^3{\rm{H}}\)?

Name	Symbol	Atomic Number	Number of Electrons	Number of Protons	Number of Protons	Number of Neutrons	Atomic structure
Protium	\({}_1^1{\rm{H}}\,{\rm{or}}\,{\rm{H}}\)	\(1\)	\(1\)	\(1\)	\(1\)	\(0\)
Deuterium	\({}_1^2{\rm{H}}\,{\rm{or}}\,{\rm{D}}\)	\(1\)	\(2\)	\(1\)	\(1\)	\(1\)
Tritium	\({}_1^3{\rm{H}}\,{\rm{or}}\,{\rm{T}}\)	\(1\)	\(3\)	\(1\)	\(1\)	\(2\)

What do You Mean by Isotope Effect?

The \(3\) isotopes of hydrogen, i.e., protium, deuterium and tritium, have the same atomic number and electronic configuration, i.e., \(1\, {{\rm{s}}^1}\;{\rm{or}}\;{\rm{K}}\left( 1 \right)\) Therefore, have similar chemical properties. But owing to their different masses, the rates or equilibrium constant of these reactions are different.

For example, the reaction between protium and chlorine is \(13.4\) times faster than between deuterium and chlorine; protium is adsorbed more rapidly than deuterium on activated charcoal. Similarly, electrolysis of ordinary water \(({{\rm{H}}_2}{\rm{O}})\) occurs more rapidly than that of heavy water \(({{\rm{D}}_2}{\rm{O}}).\)

The type of difference in the properties of isotopes due to their difference in atomic masses is called the isotope effect. Due to different masses, the physical properties of isotopes are quite different.

Which Isotopes of Hydrogen are Synthesised?

The synthesised isotopes of hydrogen are \({}_1^4{\rm{H}},\;{}_1^5{\rm{H}},\;{}_1^6{\rm{H}}\) and \({}_1^7{\rm{H}}{\rm{.}}\) All these isotopes are radioactive. 

The isotope \({}_1^4{\rm{H}}\) has one proton and three neutrons in the nucleus. It is synthesised by bombarding tritium with fast-moving deuterium nuclei.

The isotope \({}_1^5{\rm{H}}\) has one proton and four neutrons in the nucleus. It is synthesised by bombarding tritium with fast-moving tritium nuclei.

The isotope \({}_1^6{\rm{H}}\) has one proton and five neutrons in the nucleus. 

The isotope \({}_1^7{\rm{H}}\) has one proton and six neutrons in the nucleus. It is synthesised by bombarding hydrogen with helium-\(8\) atoms.

Summary

Isotopes are defined as atoms with the same number of protons and a different number of neutrons. The three isotopes of hydrogen are Protium, Deuterium, and Tritium. All the three isotopes of hydrogen have the same atomic number \(\left( {{\rm{Z}} = 1} \right).\) However, they have different mass numbers, and that is why they differ from one another in the number of neutrons in the nucleus. The atomic masses of protium, deuterium, and tritium are \(1.008, 2.014\) and \(3.016,\) respectively.

FAQs on Isotopes of Hydrogen

Q.1. Why is hydrogen called protium?
Ans: Protium \(\left( {{}_1^1{\rm{H}}} \right)\) is the abundant isotope of hydrogen with \(99.985\%.\) Hydrogen is called protium because it has only one proton and neutrons are absent in the nucleus

Q.2. What does isotope mean?
Ans: Isotopes are the different atoms of the same element which have the same atomic number but different mass numbers. Isotopes are also defined as atoms with the same number of protons and a different number of neutrons.

Q.3. Which isotope of hydrogen is unstable?
Ans: Tritium \(\left( {{}_1^3{\rm{H}}\,{\rm{or}}\,{\rm{T}}} \right)\) is the unstable isotope of carbon. It is radioactive with a short half-life of \(12.33\) years. It decays by \({\rm{\beta }}\)-emission with no \({\rm{\alpha }}\) -radiations.

Q.4. What are the \(3\) isotopes of hydrogen?
Ans: The three isotopes of hydrogen are Protium \(\left( {{}_1^1{\rm{H}}} \right),\) Deuterium \(\left( {{}_1^2{\rm{H}}\,{\rm{or}}\,{\rm{D}}} \right)\) and Tritium \(\left( {{}_1^3{\rm{H}}\,{\rm{or}}\,{\rm{T}}} \right).\)

Q.5. What are the masses of 3 common isotopes of hydrogen?
Ans: The three common isotopes of hydrogen are protium, deuterium and tritium. Its atomic masses are \(1.008, 2.014\) and \(3.016,\) respectively.

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