It contains two more neutrons than the usual form of carbon, carbon-12, and is radioactive.

H contains one proton and three neutrons in its nucleus.

The rare carbon-14 (14C) isotope contains eight neutrons in its nucleus.

Nucleus: the central part of an atom that contains protons and neutrons.

Another heavier version, or isotope, of sulfur contains two additional neutrons.

Its nucleus contains 17 protons and 20 neutrons for a total of 37 nucleons.

Calcium-48 is a rare isotope of calcium containing 20 protons and 28 neutrons.

Beryllium's most common isotope is Be, which contains 4 protons and 5 neutrons.

The other common isotope is at 19.78%, which contains 5 protons and 5 neutrons.

But among these atoms are small numbers of another isotope, carbon-13, whose nucleus contains six protons and seven neutrons.