Hydrogen

Hydrogen is a gas at room temperature, found in Group 1. It has an atomic number of 1 and an atomic weight of 1.00794. Hydrogen is the most abundant element in the universe with a proton and electron number of one. One of the hydrogen's most exciting features is that it exists in the form of 3 isotopes, different in their atomic composition: protium, deuterium and tritium^[1].

Isotopes

¹H (protium) has an atomic mass of 1.007825032241(94) u. Its core consists of only one proton and does not have any neutrons. It is the most common isotope of hydrogen which makes up to 99.98% of all hidrogen atoms found in nature.

²H (deuterium) has an atomic mass of 2.01410177811(12) u. It is another stable atom of hydrogen which core has one proton and one neutron. It is the second most common isotope of hydrogen found in nature but it makes up to only 0.0026 – 0.0184% of all atoms.

³H (tritium) has an atomic mass of 3.01604928199(23) u. Its nucleus contains one proton and two neutrons. It is a radioactive atom which has the half-life of 12.32 years. It naturaly occurs in nature because of interaction of cosmic rays with gases found in the atmosphere^[2].

References:

1. ↑ Thomas Jefferson National Accelerator Facility - Office of Science Education (2010)It's Elemental - The Element Hydrogen [Online]. Available from: http://education.jlab.org/itselemental/ele001.html [Accessed: 25 November 2010]
2. ↑ M.Wang; G.Audi; F.G.Kondev; W.J.Huang; S.Naimi; X.Xu (2017). "The Ame2016 atomic mass evaluation (II). Tables, graphs, and references" (PDF)