The atomic mass of an atom or group of atoms is the sum of several protons, neutrons, and electrons. Electrons have a small weight compared to protons or neutrons, so a large number of electrons are not involved in the calculation. In terms of the element, the atomic mass is associated with the median mass of isotopes naturally occurring in relation to the atomic mass of 12C. One atom is given exactly the relative atomic mass of 12 atoms. Atomic mass is not reported in units. The atomic mass formula is shown below.