page_title: Atomic mass
hierarchical_section_title: Atomic mass
context_page_description: The atomic mass is the mass of an atom. Although the SI unit of mass is kilogram, the atomic mass is often expressed in the non-SI unit dalton where 1 dalton is defined as ​¹⁄₁₂ of the mass of a single carbon-12 atom, at rest. The protons and neutrons of the nucleus account for nearly all of the total mass of atoms, with the electrons and nuclear binding energy making minor contributions. Thus, the numeric value of the atomic mass when expressed in daltons has nearly the same value as the mass number. Conversion between mass in kilograms and mass in daltons can be done using the atomic mass constant.
  The formula used for conversion is:
  where is the molar mass constant, is the Avogadro constant and is the experimentally determined molar mass of carbon-12.
  The relative isotopic mass can be obtained by dividing the atomic mass mₐ of an isotope by the atomic mass constant mᵤ yielding a dimensionless value. Thus, the atomic mass of a carbon-12 atom is 12 Da, 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.
context_section_description: The atomic mass (mₐ or m) is the mass of an atom. Although the SI unit of mass is kilogram (symbol: kg), the atomic mass is often expressed in the non-SI unit dalton (symbol: Da, or u) where 1 dalton is defined as ​¹⁄₁₂ of the mass of a single carbon-12 atom, at rest. The protons and neutrons of the nucleus account for nearly all of the total mass of atoms, with the electrons and nuclear binding energy making minor contributions. Thus, the numeric value of the atomic mass when expressed in daltons has nearly the same value as the mass number. Conversion between mass in kilograms and mass in daltons can be done using the atomic mass constant .
  The formula used for conversion is:
  where  is the molar mass constant,  is the Avogadro constant and  is the experimentally determined molar mass of carbon-12.
  The relative isotopic mass (see section below) can be obtained by dividing the atomic mass mₐ of an isotope by the atomic mass constant mᵤ yielding a dimensionless value. Thus, the atomic mass of a carbon-12 atom is 12 Da, 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 usually substances are not isotopially pure, it is convenient to use the elemental atomic mass which is the average (mean) 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.
  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 binding energy mass loss (as per E = mc²).
caption: 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.