Atomic mass molecular masses by comparing the vapor density of a collection of gases with molecules containing one or more of the chemical element in question. In the 20th century, until the 1960s chemists and physicists used two different atomic-mass scales. The chemists used a "atomic mass unit" (amu) scale such that the natural mixture of oxygen isotopes had an atomic mass 16, while the physicists assigned the same number 16 to only the atomic mass of the most common oxygen isotope (O, containing eight protons and eight neutrons). However, because oxygen-17 and oxygen-18 are also present in natural oxygen this led
Atomic mass Atomic mass The atomic mass ("m") is the mass of an atom. Its unit is the unified atomic mass units (symbol: u, or Da) where 1 unified atomic mass unit is defined as of the mass of a single carbon-12 atom, at rest. For atoms, the protons and neutrons of the nucleus account for almost all of the mass, and the atomic mass measured in u has nearly the same value as the mass number. When divided by unified atomic mass units or daltons to form a pure numeric ratio, the atomic mass of an atom becomes a dimensionless value