Answer :

The arrangement of these elements is known as Dobereiner triad. This triad is a group of three elements.


In 1817, Dobereiner suggested relationship between properties of elements and their atomic weights. The atomic weight of the middle element is approximately equal to the average weights of other two elements.


One example of such as set of elements is lithium, sodium and potassium.


Here, atomic weight of sodium is average weights of the lithium and potassium.


7 (lithium) + 39 (potassium)/2 = 23 (sodium)


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