Basic chemistry

Atomic Weight Definition and Examples in Chemistry

Atomic weight unit

Definition of Atomic Weightwhat is Atomic Weight

The atomic weight is the average mass of the atoms of an element, calculated using the relative abundance of isotopes in a natural element. It is the weighted average of the masses of natural isotopes.

The basis for the atomic weight unit

Before 1961, a unit of atomic weight was based on 1/16 (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 to 12 atomic mass units. Unity is dimensionless.

Also known as Atomic mass is used interchangeably, although the two terms do not mean exactly the same thing. Another problem 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 used since 1808, so most people don’t really care about the problems, but to reduce confusion, atomic weight is more commonly known as relative atomic mass.

Abbreviation: The usual abbreviation for atomic weight in texts and references is to wt or to. wt.


  • The atomic mass of carbon is 12.011; the atomic mass of hydrogen is 1.0079.
  • The atomic weight of boron samples taken on Earth is between 10.806 and 10.821.
  • For synthetic elements, there is no abundance of natural isotopes. Thus, for these elements, the total number of nucleons (sum of the number of protons and neutrons in the atomic nucleus) is generally quoted in place of the standard atomic weight. The value is given in parentheses so that we understand that it is the number of nucleons and not a natural value.

Some General 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 1/12 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 almost identical to the mass number.

The atomic mass is designated by the symbol m a.

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

Standard Atomic Weight – This is the predicted atomic weight or the relative atomic mass of a sample of an element in the Earth’s crust and the atmosphere. It is an average of the relative isotopic masses of an element from samples taken from all over the Earth.

This value is therefore likely to change as new sources of elements are discovered. The standard atomic weight of an element is the value cited for the atomic weight on the periodic table.

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