The mass of an atom or molecule on a scale where the mass of a carbon-12 (¹²C) atom is exactly 12.0 atomic mass units. The mass of any atom is approximately equal to the total number of its protons and neutrons multiplied by the atomic mass unit, u = 1.6605387 × 10⁻²⁴ gram. (Electrons are much lighter, about 0.0005486 u.) No atom differs from this simple formula by more than 1%, and stable atoms heavier than helium all lie within 0.3%.