Atomic Weight of Magnesium, History

The atomic weight of magnesium, which has always been recognised as a divalent element, has been determined by the following methods: -

1. The conversion of the oxide into the sulphate.
2. The ignition of the sulphate to the oxide.
3. The precipitation of magnesium sulphate with barium chloride.
4. The precipitation of magnesium chloride with silver nitrate.
5. The ignition of the oxalate into the oxide.
6. The ignition of the carbonate into the oxide.
7. The conversion of the metal into the oxide.

In 1884 Marignac critically reviewed the determinations of the atomic weight of magnesium, and obtained the ratio

MgO:MgSO₄ = 100:298.27

as the mean of ten conversions of the pure oxide into the sulphate, and the ratio

MgO:MgSO₄ = 100:298.31

as the mean of twelve ignitions of the sulphate into the oxide.

Using the atomic weights O = 16 and S = 32.065, these results calculate to atomic weights for magnesium of 24.38 and 24.37 respectively. Burton and Vorce, in 1890, obtained the ratio

Mg:MgO = 100:165.877

by converting the carefully purified metal into the nitrate and igniting to the oxide. This result, which is the mean of ten determinations, gives an atomic weight of 24.287 for magnesium, if O = 16.

Dumas, in 1859, as the mean of eleven determinations, obtained the ratio

MgCl₂:2Ag = 100:225.91

by estimating the silver after precipitating magnesium chloride with silver nitrate. Using atomic weights Cl = 35.457 and Ag = 107.880, this result gives 24.59 for the atomic weight of magnesium. Dumas was not very confident in his result, since he found it difficult to ensure the purity of his magnesium chloride, but he decided that it was probably better to retain 12 as the equivalent of magnesium instead of the 12.5 of Berzelius, pending further investigation.

Richards and Parker, in 1897, obtained the following results. Both their atomic weights for magnesium, calculated by using O = 16, Cl = 35.456, and Ag = 107.930, and atomic weights calculated from the fundamental data used in this series, namely, Ag = 107.880 and Cl = 35.457, are given.

In 1909 the International Commission on Atomic Weights altered the value from the 24.36 then in use to 24.32, and this value was retained in 1925.