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Dobereiner's Triads

03rd Mar 2020 @ 2 min read

Physical Chemistry

Dobereiner's triads

Statement

The Dobereiner law of triads states the arithmetic mean of properties (like atomic weight, density) of the first and third element in a triad is nearly equal to the properties of the second element in the triad.

Explanation

In the early 19th century, modern chemistry was under a premature state. Many elements were still unknown to the world. Also, the concept of the atomic number remained undiscovered.

The law of triads was an initial effort to classify elements based on their physical and chemical properties. The law of triads was proposed by Johann Dobereiner, a German chemist, in 1829.

Johann Wolfgang Dobereiner
Johann Dobereiner (1780–1849)

In the early 1850s, Dobereiner made an attempt to group elements that shared similar properties. He arranged elements into four to five groups. Each group consisted of three elements resembling in properties. Since elements were classified into sets of three, we call the law as the law of triads. The first triad was recognized in 1817. It consisted of calcium, strontium, and barium. Other triads were found in the following years.

Dobereiner observed that properties of the middle element in a triad, especially the atomic weight and specific gravity (or density), was the averaged of the other two.

The table below lists the triads with their atomic weights.

Dobereiner's triads
TriadElementsSymbolAtomic weightAveragePosition in the periodic table
Triad 1LithiumLi6.9-Group 1
SodiumNa23.023.0
PotassiumK39.1-
Triad 2CalciumCa40.1-Group 2
StrontiumSr87.688.7
BariumBa137.3-
Triad 3ChlorineCl35.5-Group 17
BromineBr79.981.2
IodineI126.9-
Triad 4SulfurS32.1-Group 16
SeleniumSe79.079.9
TelluriumTe127.6-

From the previous table, the atomic weight of the second element in a triad approximately equals the average of the atomic weight of other two. For example, the atomic weight of lithium and potassium is 6.9 and 39.1. The average of it gives 23.0, which is the atomic weight of sodium.

The atomic weight of sodium is the mean of the atomic weight of lithium and potassium.

This calculation can also be extended to the density. Besides the physical properties, the elements in the triad also share chemical properties. In the first triads, all three elements are alkali metals and are placed in group 1 of the modern periodic table. They form hydroxides (LiOH, NaOH, KOH), chlorides (LiCl, NaCl, KCl), and hydrides (LiH, NaH, KH).

The same is true for the rest triads; every element in a triad shows similar physical and chemical properties.

Limitations

The Dobereiner law of triads was an unsuccessful attempt in the classification of elements. It got shortly replaced by its successors with the discovery of new elements. It failed to fit several existing elements with similar properties into groups of three.

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