Periodic Landscapes - Relative Atomic Mass
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Relative Atomic Mass

At the turn of the nineteenth century Dalton extended and refined Prout’s remarkable conclusion that compounds were of fixed composition by proposing that atoms of the same element had the same atomic weight. Today, following the discovery of isotopes in this century, we define an atomic weight for a blend of isotopes of an element as ‘the ratio of the average mass per atom of the element to one-twelfth the mass of an atom of 12C’. As the atomic weights measured this way are numbers without units an element’s atomic weight is more correctly referred to as its ‘mean relative atomic mass’.
The atomic weights of the elements might be expected to increase steadily with increasing atomic number and so they do, with a few exceptions which caused much confusion among those early chemists.
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