Question

The three elements A, B and C with similar properties have atomic masses X, Y and Z

respectively. The mass of Y is approximately equal to the average mass of X and Z. What is

such an arrangement called as? Give one example of such a set of elements​

Answer 1

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Dobereiner observed in a few atoms that the mass of one atom is equal to the average of two atoms. He defined almost all properties basing on atomic weight. Later on, it was proved wrong any-ways. Hence, this is called Dobereiner triad. 

Lithium has 7, sodium has 23 where as potassium has 39 as atomic masses.

23=(7+39)/2=23

Here, we could observe that sodium atomic mass is average of potassium and lithium masses. 

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