The mass assigned to a carbon-12 atom in the system of relative atomic mass is 12. The atomic mass for other atoms is then derived as a ratio of 1/12 of the atomic mass of carbon-12. The relative atomic mass system is less cumbersome than basing calculations on the actual mass of an atom, which in the case of a carbon-12 atom, is approximately 1.993x10-26 kg.
A single unit of relative atomic mass, also referred to as an amu or atomic mass unit, represents approximately 1.661x10-27 kg, or 1/12 the mass of a carbon-12 atom. Since there are 6 protons and 6 neutrons in a carbon atom, this mass is also representative of the mass of 1 proton or 1 neutron. Using this system, the relative atomic mass of oxygen, which contains 8 protons and 8 neutrons, is conveniently represented by 16 rather than by its individual mass of approximately 2.657x10-26 kg.