It is called as atomic mass to mass of an atom at rest. Meanwhile, an atom is that smallest particle corresponding to a chemical element and that as such preserves its properties. Then, it is correct that the atomic mass is considered to be total mass of protons and neutrons in a single atom in a state of rest.
It should be noted that in the International System of Units the same unit is expressed as unified atomic mass unit or Dalton, whose symbol is the u or Da, respectively.
Although atomic mass has also been designated as atomic weight, such denomination is not correct and this is because the mass is property of the body and the weight is in relation to the gravity reason why it is variable.
To know the measure of the atomic mass, what is usually done is to calculate it from the average that the different isotopes of each chemical element present, always taking into account the relative abundance that each one has.
Meanwhile, the comparison and the aforementioned measurement of the atoms is feasible thanks to the use of an instrument known as Mass spectrometer. It consists of an experimental type technique that facilitates the measurement of ions derived from molecules; the precision it offers is really high, allowing us to analyze the composition of the various intervening chemical elements and atomic isotopes, separating their nuclei and depending on the mass-charge relationship.
It is important to mention that the atomic mass of an isotope will coincide with the mass of its nucleons. Such a state of affairs is plausible because the elements are not made up of a single isotope but rather a mixture that presents a certain abundance for each one.
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