This online Average Atomic Mass Calculator finds the average atomic mass of a chemical element based on the masses of its isotopes and their natural abundance. For each isotope you need to specify its mass and percentage abundance. To add or delete an isotope you may click the “+” symbol or the “–” symbol respectively.
Average Atomic Mass
It is known that most chemical elements occur in nature as a mixture of two or more isotopes. Isotopes are two or more types of atoms that have the same number of protons and differ only in the number of neutrons in their nuclei.
For example, oxygen-16, oxygen-17, and oxygen-18 are three isotopes of the element oxygen with mass numbers 16, 17, and 18, respectively. The atomic number of oxygen is 8, which means that every oxygen atom has 8 protons so that the neutron numbers of these isotopes are 8, 9, and 10 respectively.
The atomic mass of an isotope is determined mainly by its mass number (i.e., the number of nucleons in its nucleus). Small corrections are due to the binding energy of the nucleus (the so called mass defect), the small difference in mass between the proton and neutron, and the mass of the electrons bound to the atom.
The atomic mass is commonly measured using the dalton (Da) or unified atomic mass unit (u or sometimes amu). It is defined as 1/12 of the mass of an unbound neutral atom of carbon-12 at rest and in its nuclear and electronic ground state. Note that these are non-SI units that are accepted for use with the SI.
About two-thirds of the stable elements occur naturally on Earth in the form of a few stable isotopes. To take into account the natural isotopic composition in calculations related to the atomic mass of elements, the average atomic mass is used.
The average atomic mass of an element is the weighted average of the atomic masses of the naturally occurring isotopes of that element. The weights used in this calculation are the percentage abundance of a particular isotope in nature:
where
AM is the average atomic mass,
pi is the percentage abundance of isotope i,
mi is the atomic mass of isotope i.
Our Average Atomic Mass Calculator does not specify a particular unit for measuring the mass of an atom. The masses of all isotopes can be expressed using any unit you like. The average atomic mass will be expressed in the same unit.
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