Question

Select any three elements in a

sequence from any group and

calculate the arithmatic mean of the

middle element and write whether it

proves Dobereiner’s law of triad.​

Answer 1

The elements lithium, sodium and potassium form a Dobereiner's triad. Lithium is the first element of this triad, sodium is the middle element whereas potassium is the third element of the triad. Sodium (middle element) has atomic mass 23.

According to Dobereiner,

Atomic mass of sodium = (At. mass of Li + At. Mass of K)/2  = (7 + 39)/2 = 23