# Does 12 amu really equal the mass of one carbon-12 atom? [duplicate]

Furthermore, if it were true, that would convey that 1 amu is more than $1.661 \times 10^{-24}$ grams. It doesn't work out even without considering electrons. Did we get the numbers wrong, or the mass of an atom of C-12 more than 12 amu?
• When protons and neutrons are bound together by the strong nuclear force, some energy is released -- and, since $E=mc^2$, a little mass goes with it. en.wikipedia.org/wiki/Nuclear_binding_energy – Oscar Lanzi Jun 24 '16 at 22:43
• Also note that the unified atomic mass unit $\mathrm u$ should not be confused with the obsolete atomic mass unit $\mathrm{amu}$ (which is deprecated since 1961). The atomic mass of C-12 is about $12.0005\ \mathrm{amu}$ (as used in chemistry until 1961) or $12.0000\ \mathrm u$ (by definition, since 1961). However, this small difference is not related to your actual question. – Loong Jun 24 '16 at 22:57