Question

What is Dobereiner’s law of triads? Explain with the help of one example of Dobereiner’s triads.

Answer 1

Hint : Here, the answer is based on the fact that the name triad itself says that it is a group of three which means that three elements as a group of three is arranged. Based on this fact, the correct answer can be approached.

Complete answer:
We have come across the very basic concepts of chemistry that deal with the Dobereiner’s law of triads and now we shall see what this law states and understand this concept with one example.
- Dobereiner’s law of triads states that ‘arithmetic mean of the atomic masses of first and the third element in the triad should be equal to or at least almost equal to the atomic mass of the second element that is present in the triad’.
- The elements were arranged in order of their atomic masses.
- This law by Dobereiner was also extended to other properties of the elements which were quantifiable like for example density.
- For instance, the first triad were invented in 1817 and consisted of the alkaline earth metals that are calcium, strontium and barium.
Let us see the application of law to this triad.
Atomic mass of $Ca$is 40 g/mol.
Atomic mass of strontium is 88 g/mol.
Atomic mass of barium is 137 g/mol.
According to the law, first and second element’s atomic masses has to be taken average and this is as follows,
$\dfrac{40+137}{2}=88$
Therefore, this average number is nothing but the atomic mass of strontium that is the second element.
Thus, this law follows here.
This law of triads also has demerits also, that is he was not able to prepare triads for all the known elements existing.

Note: Note that scientist Dobereiner sorted the elements into logical order which is based on their physical properties and when the first triad was invented, he proved that strontium had physical properties intermediate between calcium and barium.