Atomic mass

Atomic mass
Stylized lithium-7 atom: 3 protons, 4 neutrons, and 3 electrons (total electrons are ~1⁄4300th of the mass of the nucleus). It has a mass of 7.016 Da. Rare lithium-6 (mass of 6.015 Da) has only 3 neutrons, reducing the atomic weight (average) of lithium to 6.941.
Common symbols	ma, m
SI unit	kilogram (kg)
Other units	dalton (Da)
Intensive?	yes
Behaviour under coord transformation	scalar

Atomic mass (m_a 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.^[1] 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 = mc²).

Atomic mass is often measured in dalton (Da) or 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 m_u = m(¹²C)/12 = 1 Da, where m(¹²C) 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.

The relative isotopic mass (see section below) can be obtained by dividing the atomic mass m_a of an isotope by the atomic mass constant m_u, yielding a dimensionless value. Thus, the atomic mass of a carbon-12 atom is 12 Da by definition, but the relative isotopic mass of a carbon-12 atom is simply 12. The sum of relative isotopic masses of all atoms in a molecule is the relative molecular mass.

The atomic mass of an isotope and the relative isotopic mass refers to a certain specific isotope of an element. Because substances are usually not isotopically pure, it is convenient to use the elemental atomic mass which is the average atomic mass of an element, weighted by the abundance of the isotopes. The dimensionless (standard) atomic weight is the weighted mean relative isotopic mass of a (typical naturally occurring) mixture of isotopes.