Explanation of Isotopes

The number of neutrons in an atom's nucleus is less fixed than the number of protons: many elements have different versions, called isotopes, with different numbers of neutrons. For example, there are three isotopes of carbon, called Carbon-12, Carbon-13 and Carbon-14. The numbers refer to the mass of the atom, which is the sum of the protons and neutrons. Each of the three has six protons. Carbon-12 has six neutrons. Carbon-13 has seven neutrons and Carbon-14 has eight neutrons. Some isotope...