Relative Formula Mass

This lesson covers relative atomic mass (Ar) and relative formula mass (Mr) as required by the AQA GCSE Chemistry specification (4.3.1). These are essential skills for all quantitative chemistry calculations. You need to be able to read Ar values from the periodic table, calculate Mr for any compound, and understand what these values represent.

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Relative Atomic Mass (Ar)

The relative atomic mass (Ar) of an element is the average mass of one atom of that element compared to one-twelfth of the mass of a carbon-12 atom. It is a dimensionless quantity (it has no units) because it is a ratio.

Term	Symbol	Definition
Relative atomic mass	Ar	Average mass of one atom of an element relative to 1/12 of a carbon-12 atom
Mass number	A	Total number of protons and neutrons in the nucleus of a specific isotope