Dobereiner Triads

Dobereiner Triads were an early attempt at categorizing chemical elements in a periodic manner. They gave the earliest indication that elements can occur in families with similar properties. Their discovery set the stage for new advances in the periodic classification of elements.

History

Dobereiner’s triads were first observed by Johann Wolfgang Dobereiner, a German chemist in the 19th century. He made observations about certain alkali earth metals and their salts. They formed a group of three, hence the name ‘triad’. He noticed that they had similar properties and their atomic masses followed a pattern. Later, by 1829, he could extend this theory to certain other ‘triads’ of elements.

All this suggested that some elements shared properties and could come under specific families. 

Dobereiner Law of Triads

Dobereiner Law of Triads made the following observations:

1. Some groups of three elements (‘triads’) shared common physical properties.
2. The mean or average of the atomic weights of the heaviest and lightest elements was roughly or exactly equal to the atomic weight of the middle element.
3. Some other physical properties (e.g.. density) also showed this trend. The average of the relevant property of the heaviest and lightest elements was very close to that of the middle element. 

This lets scientists see similarities in different elements, and sort them based on similarities.

Applications of Dobereiner Triads

Dobereiner’s Triads were an early example of the periodic properties of elements. They inspired future scientists to make more attempts to categorize elements based on similar properties. 

Such examples that followed include Newland’s Law of Octaves, and Lothar Meyer’s periodic classification. Eventually, this led to Mendeleev’s Periodic table and our Modern Periodic Table. 

Examples of Dobereiner Triads

Here we present the original Dobereiner Triads examples.

Alkaline Earth Triad

This was the first triad to be discovered.

This consists of the elements calcium (\(\mathrm{Ca}\)), strontium (\(\mathrm{Sr}\)) and barium (\(\mathrm{Ba}\)).

Below their atomic masses are presented, in increasing order, where the pattern becomes clear.

As we can see, the actual atomic weight of Strontium, the middle element, is \(87.6 \text{u}\). If we take the average of the first and last element’s atomic weights, we get, \(\frac{(40.1+137.3)}{2} = 88.7 \text{u}\) which is very close to \(87.6 \text{u}\).

Alkali Metal Triad

This consists consists of lithium (\(\mathrm{Li}\)), sodium (\(\mathrm{Na}\)) and Potassium (\(\mathrm{K}\)).

Tabulating their atomic masses, we see:

Clearly, the average of the atomic masses of lithium and potassium is \(\frac{(6.94+39.1)}{2} = 23.02 \text{u}\). This is almost exactly the atomic mass of sodium, \(23.02 \text{u}\).

Halogen Triad

The halogen elements also show these trends. They are also a Dobereiner triads example.

Taking the average of the atomic masses of the heaviest and lightest elements, we have, \(\frac{(35.4+126.9)}{2} = 81.2 \text{u}\). This is reasonably close to the atomic weight of bromine, which is \(79.9 \text{u}\). 

Chalcogen Triad

The chalcogen triad consists of elements Sulphur (\(\mathrm{S}\)), Selenium (\(\mathrm{Se}\)) and Tellurium (\(\mathrm{Te}\)) from the oxygen family. 

Again, the average of the first and light element atomic masses gives us \(79.9 \text{u}\), very close to the atomic mass of \(\mathrm{Se} , 78.9 \text{u}\).

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