Force is equal to mass times acceleration. If we apply a known force to our atom and measure its acceleration, we can know its mass.

For example, a helium atom has an atomic mass of 4, but an atomic number (or positive charge) of 2. Since electrons have almost no mass, it seemed that something besides the protons in the nucleus ...

So, now instead of carbon listed as being 12.0107 atomic mass units with a measurement uncertainty of about 0.0008, it has an official atomic weight of [12.0096; 12.0116], where the brackets and ...