We can think that to determine the mass of an atom we only have to add the masses of the particles that constitute it (protons, neutrons and electrons). However, when the nucleus is formed, a significant amount of energy is released, called nuclear energy, which implies a decrease in the mass of the nucleus with respect to the sum of the masses of the particles that compose it.

Therefore, the determination of atomic masses must be done experimentally. First, the carbon-12 atom is chosen as the standard and assigned a mass of 12 atomic mass units (amu). Next, the mass of the remaining atoms is determined with respect to carbon-12 using a mass spectrophotometer.

For example, data from a mass spectrum indicate that the ratio of the masses of \(^{16}O\) to \(^{12}C\) is 1.33291. That is, oxygen has a mass 1.33291 times greater than carbon, the mass of oxygen will be given by: \(1.33291\;x\;12=15.9949\;uma\)