Mass number number of protons in a nucleus, and thus uniquely identifies an element. Hence, the difference between the mass number and the atomic number gives the number of neutrons ("N") in a given nucleus: . The mass number is written either after the element name or as a superscript to the left of an element's symbol. For example, the most common isotope of carbon is carbon-12, or , which has 6 protons and 6 neutrons. The full isotope symbol would also have the atomic number ("Z") as a subscript to the left of the element symbol directly below the mass number:
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