Difference Between Atomic Weight and Atomic Mass

Atomic Weight vs Atomic Mass

Atomic mass and atomic weight are interchangeably used by most people. However, they bear different meanings, and it causes a significant error in bulk material calculations if these two are taken as one.

Atomic Mass

Atoms are mainly composed of protons, neutrons and electrons. Atomic mass is simply the mass of an atom. In other words, it is the collection of masses of all the neutrons, protons and electrons in a single atom, specifically, when the atom is not moving (rest mass). Rest mass is taken because; according to the fundamentals of physics, it has been shown that when atoms are moving at very high velocity the masses increase. However, the mass of electrons is considerably very small compared to the masses of protons and neutrons. So, we can say that the electrons’ contribution to an atomic mass is less. Most of the atoms in the periodic table have two or more isotopes. Isotopes differ from each other by having a different number of neutrons, even though they have the same proton and electron amount. Since their neutron amount is different, each isotope has a different atomic mass.

Moreover, the masses of atoms are extremely small, so we cannot express them in normal mass units like grams or kilograms. For our purposes, we are using another unit call atomic mass unit (amu) to measure the atomic mass. 1 atomic mass unit is the one twelfth of the mass of a C-12 isotope. When a mass of an atom is divided by the mass of one twelfth of the mass of a C-12 isotope, its relative mass is obtained. However, in the general use when we say the relative atomic mass of an element, we mean their atomic weight (because it is calculated considering all the isotopes).

Atomic Weight

As mentioned above, there can be several isotopes for a single element. Atomic weight is the average weight calculated considering all the masses of isotopes. Each isotope is present in the environment in different percentages. When calculating the atomic weight, both isotope mass and their relative abundances are taken into consideration.

IUPAC defines the atomic weight as follows:

“An atomic weight (relative atomic mass) of an element from a specified source is the ratio of the average mass per atom of the element to 1/12 of the mass of an atom of 12C.”

The weights given in the periodic table are calculated like this, and they are given in relative atomic mass.

What is the difference between Atomic Weight and Atomic Mass?

- Atomic mass is the mass of a single atom, which is its collective mass of neutrons, protons and electrons.

- Atomic weight is the average weight of an element, with respect to all its isotopes and their relative abundances.


Atomic mass or atomic weight doesn’t give a different meaning if there is only one isotope of an element. But, if there are more isotopes, then the mass of the most abundant isotope contributes more towards the atomic weight. For example, Cl-35 natural abundance is 75.76% and Cl-37 abundance is 24.24%. The atomic weight of Chlorine is 35.453 (amu), which is closer to the mass of the Cl-35 isotope.