https://doi.org/10.15255/KUI.2018.042
Published: Kem. Ind. 70 (5-6) (2021) 319–326
Paper reference number: KUI-42/2018
Paper type: Professional paper / Chemistry in Teaching
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What is Relative Atomic Mass?

N. Raos

Abstract

There are two basic understandings of relative atomic mass. The first (“atomic weight”) stems from the law of constant and multiple mass proportions and has no reference to the size and the absolute mass of atoms. “Atomic weights” were defined and measured in the 19^th Century, and at that time atoms and molecules were just hypothetical particles. However, in the 20^th Century, it became possible to measure the number and mass of atoms, and relative atomic mass was defined in reference to the mass and number of atoms, i.e. Avogadro constant and mole. This also resulted in the change of reference unit, A_r(H) = 1 or A_r(O) = 16, for A_r(¹²C) = 12, and the way of teaching of stoichiometry (calculations via n_A instead of using proportionality principle). This article also describes the history of “atomic weight” determination by gravimetric analysis, especially the work of J. J. Berzelius and J. S. Stas.

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Keywords

history of chemistry, chemistry education, Dalton, Berzelius, Richter, Stas