Isotopes: Atoms of the same element that contain different numbers of neutrons

Mass number: The number of protons and neutrons in one atom of an element. The mass number is always a whole number.

Relative atomic mass: The weighted mean mass of an atom of an element compared to \( \frac{1}{12} \) of the mass of an atom of carbon-12.

Relative atomic mass = \( \frac{\text{Total mass of the atoms}}{\text{Total number of atoms}} \)

Relative formula mass: The sum of the relative atomic masses of all the atoms in the empirical formula of an molecule

Relative molecular mass: The sum of the relative atomic masses of all the atoms in a molecule

Relative isotopic mass: The mass of an individual atom of a particular isotope relative to \( \frac{1}{12} \) of the mass of an atom of carbon-12. This is not likely to be a whole number.