Isotopes are atoms of the same element that have the same number of protons (atomic number), but different numbers of neutrons. Because mass number is the sum of protons and neutrons in the nucleus of an atom, isotopes of the same element are named according to their mass numbers. For example, carbon-12 is the isotope of carbon with a mass number of 12, and carbon-14 is the isotope of carbon with a mass number of 14. Both isotopes have 6 protons, but carbon-12 has 6 neutrons and carbon-14 has 8 neutrons.
Another way to represent different isotopes is called isotopic notation, or nuclear notation. This type of notation involves writing the chemical symbol for the element, and then writing the mass number as a superscript to the left of the symbol, and the atomic number (no. of protons) as a subscript to the left of the symbol. For example, carbon-12 is written as