Hydrogen is the simplest, lightest and the most abundant element in the universe with one proton in its nucleus. This highly reactive element makes up about 75% of the total mass in the universe. Hydrogen occurs naturally in the form of three of its isotopes namely, Protium 1H, Deuterium 2H, and Tritium 3H. The most common and stable isotope is Protium, which naturally and very strongly bonds with itself as H2. Besides these isotopes, there are synthetic isotopes from 4H to 7H.


Symbol H
Atomic number 1
Atomic mass 1.007825 g/mol
Melting point -259.2 °C
Boiling point -252.8 °C
Density 0.0899×10-3 g/cm3 at STP
Isotopes 3

Frequently Asked Questionsedit

Why does Hydrogen lack a neutron?edit

The most common isotope of Hydrogen, Protium (1H) is the one that lacks a neutron. Other isotopes like Deuterium and Tritium have neutrons in them. The reason why Protium is abundant and stable is that in the very beginning, the universe had free protons, electrons, and neutrons. Due to the short mean lifetime of the free neutrons, they decayed before they could form enough Deuterium atom. So Protium atoms formed in abundance, being the simplest stable atomic configuration ever.

Why doesn't Hydrogen in Sun fuse together all at once?edit

The fusion of Hydrogen in stars is governed by the probability of the hydrogen atoms colliding with enough energy to fuse into Helium. Most hydrogen atoms in our Sun repels due to the electrostatic repulsion between them. But with enough time, some hydrogen atoms tunnel through the electrostatic repulsion and fuse to form Helium-4. This phenomenon is known as quantum tunnelling. But the chances of that Helium-4 nucleus undergoing a beta decay and forming deuterium, which further carries on the nuclear fusion reaction, are slim. These conditions in our Sun are the key factor why all the Hydrogen in it never fuse all together at once. In the case of heavier stars, where gravity and temperature are very high, the rate of fusion could be much faster.