What is Relative Atomic Mass ?

Definition of 'Relative Atomic Mass':

The Relative Atomic Mass of an element is the mass of an 'average atom' of that element (taking into account its different isotopes and their relative proportions) compared with the mass of an atom of carbon-12.

Background Information

It is useful to understand the following in order to be able to explain relative atomic mass: atom, atomic number, element, mass number and isotope.

At the center of each atom is a nucleus that consists of two types of particles called protons and neutrons (and known collectively as nucleons). The number of protons is called the atomic number and determines which element the atom is a particle of (and therefore also its chemical properties). The total number of protons and neutrons in the nucleus of the atom is called the mass number and is determined by the type of atom (i.e. which element it is) and also by the number of neutrons present in that particular atom. In many cases, different atoms of the same element can have different numbers of neutrons in their nuclei (nuclei is the plural word for nucleus; one nucleus - many nuclei). The number of neutrons in an atom of a specific element therefore determines its mass number and so the isotope of which that particular atom is an example.

For example, most atoms of the element phosphorus have a mass number of 31 and so may be referred to as phosphorus-31, however, some atoms of phosphorus have an extra neutron and therefore a mass number of 32. They are therefore atoms of the isotope phosphorus-32.

Although there are three types of particles that form atoms (protons, neutrons and electrons), mass of atoms is mainly due to the numbers of protons and neutrons that make-up the atom, i.e. the mass number of the atom.

  • The atomic number of an atom is the same as the atomic number of the element of which that atom is a particle because the atomic number of an atom determines which element it is an atom of.

    however ...

  • The mass number of an atom is not the same as the mass number of an element because atoms of the same element can have different masses due to having different numbers of neutrons (i.e. exist in the form of different isotopes, which therefore have slightly different physical properties e.g. boiling points).

That is the Atomic Number of an element is meaningful, but the Mass Number 'of an element' is not meaningful.

When dealing with elements (macroscopic scale) rather than atoms (individual particles), chemists use Relative Atomic Mass because 'mass number' only applies to specific atoms of one particular isotope of the element.

Calculation of Relative Atomic Mass

In order to calculate the relative atomic mass of a sample of an element, it is necessary to know which isotopes of that element are present in the sample, and in what proportions.

The following calculation of relative atomic mass uses the example of chlorine:

Consider a sample of chlorine that contains the isotopes chlorine-35 and chlorine-37 in the ratio 3:1, meaning that for every one atom of chlorine-37 there are 3 atoms of chlorine-35.

The relative atomic mass of this sample of chlorine may be calculated by taking into account the atomic masses and relative proportions of its isotopes as follows:

Isotope 1

Isotope 2




Mass Number



Relative Proportion




35 x 3 = 105

37 x 1 = 37

= 105 + 37 = 142

= 142 / (3+1)

Relative Atomic Mass of this sample

= 142 / 4 = 35.5

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