Strontium triad relationships

In 1817 Dobereiner found that if certain elements were combined with oxygen in binary compounds, a numerical relationship could be discerned among the equivalent weights of these compounds. Thus when oxides of calcium, strontium, and barium were considered, the equivalent weight of strontium oxide was approximately the mean of those of calcium oxide and barium oxide. The three elements in question, strontium, calcium, and barium were said to form a triad. 

As the number of elements increased, so did attempts to organize them into meaningful relationships. Johann Dobereiner (1780-1849) discovered in 1829 that certain elements had atomic masses and properties that fell approximately mid-way between the masses and properties of two other elements. Dobereiner termed a set of three elements a triad. Thus, chlorine, bromine, and iodine form a triad Dobereiner proposed several other triads (lithium-sodium-potassium, calcium-strontium-barium). Dobereiner recognized that there was some sort of relationship between elements, but many elements did not fit in any triad group, and even those triads proposed displayed numerous inconsistencies. 

The triad in Table 3.2 shows a relationship among the densities of three metals that is true for many triads. The density of strontium (2.60 g/mL) is near the average of the densities of calcium (1.55 g/mL) and barium (3.62 g/mL) ... 

That the elements [Hissessed relationships uf a different Hurt was shown soon after the establishment of Dalton s Atomic Theory. As early as 1817, Dnehereiner culled attention to the fact that strontium hud an atomic weight which was very close to the mean of the values for calcium and barium, while these three elements showed dose similarity in both physical mid chemical properties, Tatter he also showed that there are other triads in which the same general relationship exists, such... 

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