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Isotopes of Hydrogen


The number of protons in a nucleus of an atom determines the element's atomic number. Isotopes are defined as members of an atomic family of the elements, in which, all of the isotopic elements have same number of protons but different numbers of neutrons. In easier terms, the mass number of isotopes of an element varies but the atomic number remains same for each one of them. Isotopes of an element share almost the same chemical properties, but due to difference in their mass numbers the physical properties are not likely to be similar.

The element symbol of an isotope is: AEZ.


  • A = number of protons + number of neutrons = mass number
  • Z = number of protons = number of electrons = atomic number

Isotopes of Hydrogen:

Hydrogen is a chemical element which is non-metallic in nature, its atomic symbol is (H) and its atomic number is 1. The number of neutrons in a neutral hydrogen atom is 0. Therefore, the number of protons, neutrons and electrons present in a neutral hydrogen atom are 1, 0, and 1 respectively. It is thus, an electrically neutral atom as it contains a single positively-charged proton and a single negatively charged electron bound to the nucleus by the Coulomb force. It is the lightest element and acquires the topmost position in the periodic table. It is kept along with the elements of group 1 but slightly separated as it has distinctive feature. Hydrogen isotopes occupy the same position in the periodic table as each has one proton.

Hydrogen has three naturally found isotopes; protium, deuterium and tritium. However, it can also have a number of synthetic isotopes. Each one of these categories is discussed below:

  1. Naturally occurring isotopes of hydrogen:
    Hydrogen atom has three naturally occurring isotopes; protium, deuterium and tritium.
    They are denoted by 1H1, 2H1, and 3H1, respectively, where 1, 2 and 3 represents the mass number of the isotopic element. Hydrogen is the only element whose isotopes have different names that are commonly used today. As we see, 1H1 (or hydrogen-1) stands for Protium, 2H1 (or hydrogen-2) stands for Deuterium and 3H1 (or hydrogen-3) stands for Tritium. The isotope 1H have no neutrons within it. 1H and 2H are comparatively stable isotopes of hydrogen, whereas 3H is known to have a half-life of 12.32 years, which makes it highly unstable in nature.
  2. Synthetic isotopes of hydrogen:
    Heavier isotopes of hydrogen also exist, all of which are synthetic and have a half-life of less than one zepto-second ( equals to 10?21 s), that's why they are not in much use but a number of scientific studies are going on for understanding them in a better way. From all of these, 5H1 is the least stable isotope for hydrogen, while 7H1 is the most stable one. These isotopes are synthesized in laboratories but are not observed in nature.

Nomenclature of isotopes of hydrogen:

The symbols D and T are prevalent in use for deuterium and tritium respectively. The International Union for Pure and Applied Chemistry (IUPAC) accepts the D and T symbols, but recommends using standard isotopic symbols only which are written as 2H1 and 3H1. This is to avoid any confusion in the alphabetic arrangement and sorting of chemical formulas. During the early study of radioactivity, some other heavy radioactive synthetic isotopes were also given names, but such names are rarely used today, their application is observed only in the field of scientific research including radioactivity platform related sciences and technologies.

Occurrence of Isotopes of Hydrogen:

  1. Occurrence of Protium:
    Protium has a total abundance in the oceans of ~156.25 ppm, which accounts for about 0.0156 percent of all hydrogen present on earth. The proton was never observed to decay and thus, hydrogen-1 (Protium) is considered as a stable isotope, this clearly describes the relative abundance of this isotope in nature.
  2. Occurrence of Deuterium:
    Fortunately, deuterium is commonly found in our environment. About 1 out of every 5,000 hydrogen atoms in seawater is deuterium that shows our oceans contain a huge amount of deuterium. It is produced by the rare process of cluster decay, and occasional absorption of naturally occurring neutrons by light hydrogen.
  3. Occurrence of Tritium:
    Tritium is produced naturally in the upper atmosphere when cosmic rays strike nitrogen molecules in the air. It is also produced in the sun as a subset of the proton-proton chain of fusion reactions. Although a steady stream of the tritium near the surface of the sun is ejected out into space, on the solar wind, much larger streams are ejected out into space during solar flares and prominence. It is more energetic than its solar wind counterpart, so, tritium produced in this way injected directly into the earth's upper atmosphere. It is also produced during nuclear weapons explosions, and as a byproduct in nuclear reactors. Its most common form is Tritiated water (HTO). This happens when a tritium atom replaces a hydrogen atom in commonly used water (H2O) to form HTO. HTO has the same chemical properties as water and is also odorless and colorless.

Properties of Isotopes:

Physical and Atomic Properties of Protium:

The atomic number of protium is 1 as it has one proton. The mass number of protium is also 1 since there are no neutrons in the nucleus of protium. The atomic mass of protium is around 1.00794 amu. The symbol for protium is 1H. The electron configuration of protium is 1s1. Protium can be found in nature as a diatomic gaseous form or as hydrogen in H2O molecule. The bond between two atoms in the diatomic molecule has a higher bond dissociation enthalpy. This is mainly because these atoms are minute and they have complete electron configurations in the only orbital (s orbital) in their diatomic molecule form. Commonly observed physical and atomic properties of protium are enlisted in the table given below:

Physical Property Value
Atomic mass 1.0079 amu
Melting Point 13.957K
Boiling Point 20.39K
Enthalpy of Vaporization 0.904KJ/mol
Enthalpy of Fusion 0.117KJ/mol
Density 0.09g/L
Critical Pressure 12.98 bar
Critical Temperature 33.19K
Nuclear spin quantum number
Enthalpy of Dissociation 435.9KJ/mol

Physical and chemical properties of Deuterium:

This isotope of hydrogen has one proton, one neutron, and one electron. Its nucleus contains one proton and one neutron. Its symbol is 2H; atomic number is 1 and the mass number is 2. The atomic mass is 2.014 amu. This is also a stable isotope of hydrogen but is less abundant. The abundance of deuterium in the earth's crust has been calculated as 0.015%. It is not radioactive since deuterium is stable with one proton and one neutron in its nucleus. Common physical and atomic properties of deuterium are enlisted below:

Physical Property Value
Atomic mass 2.0141 amu
Electron affinity 0.754 eV
Boiling Point -250o C
Enthalpy of Vaporization 216 cal/mol
Enthalpy of Fusion 28 cal/mol
Density 0.0310 g/cm3
Enthalpy of Dissociation 105.97 kcal/mol
Ionization Potential 13.600 eV
Nuclear spin quantum number 1

Physical and Chemical Properties of Tritium:

This isotope of hydrogen has one proton, two neutrons, and one electron. Its symbol is 3H; atomic number of tritium is 1; atomic mass of tritium is 3. The atomic mass can be given as 3.016 amu. It is radioactive as it contains more neutrons compared to the number of protons. Tritium often undergoes beta decay, which produces Helium-3 along with a large amount of energy.

The half-life of tritium has been calculated as 12.32 years. However, it is present in very less amount on the earth's crust. Common physical and atomic properties of tritium are enlisted in the table given below:

Physical Property Value
Atomic mass 3.0160 amu
Half life 12.32 years
Beta emission average energy 5.685 0.008 keV
Average range in water 0.56 ?m
Density 2.589 Ci/cm3

Applications of Isotopes of Hydrogen:

1. Applications of Protium:

Protium is the most common form of hydrogen isotope, with an abundance of more than 99.98%. It is given this name because of the fact that the nucleus of this isotope consists of only a single proton, it is thus given the descriptive formula 1H. It is unique in this sense that among all stable isotopes it is having no neutrons. Protium is a rare element that is used in a variety of ways. The fields of utility of protium are as follows:

  • Protium contains pantoprazole which acts as an active ingredient, useful in many manufacturing units.
  • Protium is a selective "proton pump inhibitor," a medication that reduces the amount of stomach acid produced. It is thus, used to treat stomach and intestine acid-related illness.
  • Protium being a non-toxic, colorless, odorless and tasteless liquid is mostly used as a fuel source. It is also used in the manufacturing of various chemicals and plastics.
  • It is used in nuclear reactors to create energy, in light bulbs to create light, and in batteries to create power. It is also used in a variety of medical applications.
  • Protium is used in nuclear reactors to create energy. The element is used to start the nuclear reaction and to keep it going. It is also used in nuclear weapons to create the explosion.
  • Protium is used in light bulbs to create the filament that generates the light.
  • Protium is used in batteries to create power. The element is used to create the anode and cathode that ultimately generates the power.
  • Protium is also used in a variety of medical applications. It is used to create contrast agents for MRI scans, to diagnose and treat diseases, and to study the human body.

2. Applications of Deuterium:

Deuterium contains one proton and one neutron in the nucleus. The history of deuterium dates back to the Big Bang, it is thought to have been produced at that time and has been observed to be enduring since then. It is not radioactive, and does not represent a significant toxicity hazard. The fields of utility of deuterium are as follows:

  • Heavy water is the water enriched in molecules that include deuterium instead of normal hydrogen.
  • Heavy water is scientifically used as a neutron moderator and coolant for nuclear reactors.
  • Heavy water is also used in place of normal water in IR (infrared) spectroscopy.
  • Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for 1H-NMR spectroscopy.
  • Deuterium is also a potential fuel for commercial activities of nuclear fusion.

3. Applications of Tritium:

Tritium is a radioactive form of hydrogen that exists naturally, formed in the atmosphere when cosmic rays interfere with air molecules. Therefore, tritium is present worldwide in very low concentration in groundwater. This is also formed as a by-product of the nuclear power plants producing electricity. The fields of utility of tritium are as follows:

  • It is widely used in glow-in-the-dark lighting and signs. Tritium gas when reacts with phosphor creates luminescence. The light source does not require electricity or electrical wiring, this makes it ideal for exit signs as well as for emergency lighting in commercial buildings, airplanes and for airport runway lights.
  • Tritium is also used as a tracer in biomedical and academic research. In some countries it is used as fuel in thermonuclear weapons. In the future, can also be used to generate electricity in fusion reactors.


Hydrogen isotopes occupy the same position in the periodic table because all of them contains one proton each. Their chemical behavior is similar because of similarity in electronic configuration. However, considerable differences in their physical properties which are mass-dependent are observed. A summarized table is as given below:

Name Chemical symbol Nuclear symbol Atomic mass Natural abundance (%) Natural abundance (x : H ratio)
Protium H 1H1 1.007 825 99.985% 1:1
Deuterium D 1H2 2.014 101 0.015% 1:6,600
Tritium T 1H3 3.016 049 26 a Very low 1:1017

Out of the three natural isotopes of hydrogen, only tritium is radioactive in nature which emits low-energy beta particles. As the electronic configuration of isotopes is similar, they all have same chemical properties. But they have a difference in their rates of reaction, this happens because of the different bond disassociation enthalpies which acts as a function of mass. They have different physical properties because of the large variations in mass.

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