What is the Relative Atomic Mass of a Chemical Element?

The relative atomic mass of a chemical element can be defined as the ratio of the atomic mass constant to the average mass of all the atoms of a specific element in a given sample. This dimensionless quantity is also referred to as atomic weight. Relative atomic mass is often denoted by the symbol Ar. The value of this physical quantity is said to be ‘relative’ and ‘dimensionless’ because it considers the ratio between two different masses.

For any given sample, the atomic weight (or the relative atomic mass) of a specific element can be determined by calculating the weighted arithmetic mean of the masses of all the atoms of the element in the sample. It is important to note that this weighted arithmetic mean must also account for the variance in the masses of the atoms in the sample due to the presence of isotopes. Therefore, the abundance of a specific isotope in the sample can directly impact the relative atomic mass of the element.

Applications of Relative Atomic Mass

Relative atomic masses are of vital importance when calculating the standard atomic weight of an element. The standard atomic weight of a chemical element can be defined as the arithmetic mean of the atomic weights (relative atomic masses) of all stable isotopes of the element, weighted by the abundance of the respective isotopes on the Earth’s surface. Standard atomic weight is often denoted by the symbol ‘Ar, standard’. 

Fun Fact:

The standard atomic weights of all the chemical elements provided on the periodic table are the values determined by the Commission on Isotopic Abundances and Atomic Weights (CIAAW) – a subdivision of the International Union of Pure and Applied Chemistry (IUPAC). 

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