Sulfur has 24 known isotopes, 4 of which are stable (meaning that they do not undergo radioactive decay). S, the common isotope of sulfur, makes up 95.0% of the natural sulfur on Earth. In the atomic symbol of S, the number 32 refers to the mass of each sulfur atom in daltons, the result of the 16 protons and 16 neutrons of 1 dalton each that make up the sulfur nucleus. The three rare stable isotopes of sulfur are S (4.2% of natural sulfur), S (0.75%), and S (0.015%). These isotopes differ from S in the number of neutrons in each atom, but not the number of protons or electrons; as a result, each isotope has a slightly different mass, but has nearly identical chemical properties.