Question

State Dobereiner’s Triads by giving one example.

Answer 1

We have to know that Dobereiner’s triads and Newland’s law of octaves were early attempts to classify elements into groups according to their properties. Since several new elements were discovered during the 18th and 19th centuries, the classification of elements into metals and nonmetals became inefficient. Several experiments were performed to determine elements with like properties and gather them together.

Complete step by step answer:
Dobereiner’s triads were groups of elements with like properties, which were found by German chemist Johann Wolfgang Dobereiner. He detected that groups of three elements (triads) can be formed where all the elements shared the same physical and chemical properties.
Dobereiner law of triads states that the average of the atomic masses of the first and third element in a triad will be roughly equal to the atomic mass of the second element in that triad. He suggested that this law can be extended for other properties of elements like density.
The first of Dobereiner’s triads was determined during the year 1817 and consisted of the alkaline earth metals calcium, strontium, and barium. Three more triads were determined in the year 1829.
For example:
The atomic mass of calcium is $40\,g/mol$.
The atomic mass of strontium is $88\,g/mol$.
The atomic mass of barium is $137\,g/mol$.
$\dfrac{{40 + 137}}{2} = 88$
The atomic weight of strontium is nearly the average of the atomic masses of calcium and barium.
Some of the other examples of Dobereiner’s triads are,
$Li\left( {7\,g/mol} \right),Na\left( {23\,g/mol} \right),K\left( {39\,g/mol} \right)$
$Cl\left( {35.5\,g/mol} \right),Br\left( {80\,\,g/mol} \right),I\left( {127\,g/mol} \right)$
$S\left( {32\,g/mol} \right),Se\left( {79\,g/mol} \right),Te\left( {128\,g/mol} \right)$

Note:
We shall discuss the some of the limitations of Dobereiner’s Triads are,
Newly invented elements did not fit into the triads.
Only 5 Dobereiner’s triads were discovered.
The discovery of new elements made Dobereiner’s triads no longer in use.
Several known elements also did not find any place in any of the triads.