Isotope electrons in the neutral (non-ionized) atom. Each atomic number identifies a specific element, but not the isotope; an atom of a given element may have a wide range in its number of neutrons. The number of nucleons (both protons and neutrons) in the nucleus is the atom's mass number, and each isotope of a given element has a different mass number. For example, carbon-12, carbon-13 and carbon-14 are three isotopes of the element carbon with mass numbers 12, 13 and 14 respectively. The atomic number of carbon is 6, which means that every carbon atom has 6 protons, so that
Relative atomic mass Relative atomic mass Relative atomic mass (symbol: "A") or atomic weight is a dimensionless physical quantity defined as the ratio of the average mass of atoms of a chemical element in a given sample to one unified atomic mass unit. The unified atomic mass unit (symbol: "u" or "Da") is defined as being of the atomic mass of a carbon-12 atom. Since both values in the ratio are expressed in the same unit ("u"), the resulting value is dimensionless; hence the value is said to be "relative". For a single given sample, the relative atomic mass of a given element