Hydrogen (H) is the first element in the periodic table.

Atomic number, Z = 1

Electronic Configuration : 1s1

Average relative atomic mass: 1.00794 amu


* Hydrogen is the most abundant element in the universe (92%). 

* It also accounts for 15.4% of all atoms in earth’s crust. 

* The elemental form is diatomic molecule, H2 and it exists as a gas at room temperature.

* However the elemental form of hydrogen is highly reactive and hence it occurs in the combined state - mostly as water.

Isotopes of hydrogen 

* There are three isotopic forms of hydrogen: Protium (1H1), Deuterium (1H2 or D), Tritium (1H3 or T). 


Isotope Symbol No. of Protons
No. of Neutrons
Mass number
(A = p + n)
Mass %
Protium 1H1 1 0 1 1.00782 99.985%
Deuterium 1H2 or D 1 1 2 2.01410 0.015%
Tritium 1H3 or T 1 2 3 3.01604 7x10-16 %


* Protium is the most abundant form of hydrogen. Since it contains only one proton and no neutron, its atomic mass is roughly equal to proton. The percent composition of protium is 99.985%. Hence the average atomic mass of hydrogen is equal to 1.00794 amu.

* Deuterium is called heavy hydrogen. It constitutes 0.015% of total hydrogen. It contains a neutron.

* Tritium has two neutrons. It is radioactive and disintegrates by β-decay. It’s half life is 12.26 years. 

* Tritium is formed in the nature due to following nuclear transformation. 

* It is used as tracer in the study of mechanism of chemical reactions. 

* It is relatively cheap and safe to work with Trituim in the laboratory. It is non toxic due to emission of low energy β-radiation (no γ-radiation).

* The isotopes of hydrogen show similar chemical properties since they possess same electronic configuration. But they show different physical properties due to different masses. 

* However the reactivity of protium is more than that of deuterium. It is due to relatively stronger D-D bond.



      Water (H2O)>


Author: Aditya vardhan Vutturi