atomic mass
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Atomic mass is the weight of an atom when it's at rest and in its lowest energy state. It matters because it helps scientists identify elements, predict how atoms will interact with each other, and understand the properties of materials.
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Atomic mass (ma or m) is the mass of a single atom. The atomic mass mostly comes from the combined mass of the protons and neutrons in the nucleus, with minor contributions from the electrons and nuclear binding energy. The atomic mass of atoms, ions, or atomic nuclei is slightly less than the sum of the masses of their constituent protons, neutrons, and electrons, due to mass defect (explained by mass–energy equivalence: E = mc).
Atomic mass is often measured in dalton (Da) (a.k.a. unified atomic mass unit (u)). One dalton is equal to +1/12 the mass of a carbon-12 atom in its natural state, given by the atomic mass constant mu = m(C)/12 = 1 Da, where m(C) is the atomic mass of carbon-12. Thus, the numerical value of the atomic mass of a nuclide when expressed in daltons is close to its mass number.