Atomic Weight Definition

With Related Terms and Examples

Graphic of an atom with nucleus and electrons in their orbits


Atomic weight is the average mass of atoms of an element, calculated using the relative abundance of isotopes in a naturally-occurring element. It is the weighted average of the masses of naturally-occurring isotopes.

What Is It Based On?

Prior to 1961, a unit of atomic weight was based on 1/16th (0.0625) of the weight of an oxygen atom. After this point, the standard was changed to be 1/12th the weight of a carbon-12 atom in its ground state. A carbon-12 atom is assigned 12 atomic mass units. The unit is dimensionless.

More Commonly Known as Relative Atomic Mass

Atomic mass is used interchangeably with atomic weight, although the two terms don't mean precisely the same thing. Another issue is that "weight" implies a force exerted in a gravitational field, which would be measured in units of force, like newtons. The term "atomic weight" has been in use since 1808, so most people don't really care about the issues, but to reduce confusion, atomic weight is more commonly known now as relative atomic mass.


The usual abbreviation for atomic weight in texts and references is at wt or at. wt.


  • The atomic mass of carbon is 12.011
  • The atomic mass of hydrogen is 1.0079.
  • The atomic weight of boron samples collected on Earth falls within a range of 10.806 to 10.821.

Synthetic Elements

For synthetic elements, there is no natural isotope abundance. So, for these elements, the total nucleon count (sum of the number of protons and neutrons in the atomic nucleus) is usually cited in the place of the standard atomic weight. The value is given within brackets so that it's understood it's the nucleon count and not a natural value.

Related Terms

Atomic Mass - Atomic mass is the mass of an atom or other particle, expressed in unified atomic mass units (u). An atomic mass unit is defined as 1/12th the mass of a carbon-12 atom. Since the mass of electrons is much smaller than that of protons and neutrons, the atomic mass is nearly identical to the mass number. Atomic mass is denoted with the symbol ma.

Relative Isotopic Mass - This is the ratio of the mass of a single atom to the mass of a unified atomic mass unit. This is synonymous with atomic mass.

Standard Atomic Weight - This is the expected atomic weight or relative atomic mass of an element sample in the Earth's crust and atmosphere. It is an average of relative isotope masses for an element from samples collected all over the Earth, so this value is subject to change as new element sources are discovered. The standard atomic weight of an element is the value cited for atomic weight on the periodic table.

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Your Citation
Helmenstine, Anne Marie, Ph.D. "Atomic Weight Definition." ThoughtCo, Apr. 5, 2023, Helmenstine, Anne Marie, Ph.D. (2023, April 5). Atomic Weight Definition. Retrieved from Helmenstine, Anne Marie, Ph.D. "Atomic Weight Definition." ThoughtCo. (accessed June 8, 2023).