In Dobereiner's triads, when three elements in a triad are written in the increasing order of atomic masses; the atomic mass of the middle element is roughly the average of the atomic masses of the other two elements. Identify ‘X’ from the given triad.
Li (6.9), Na (X), K (39.0)

15.3

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23.0

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19.0

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45.9

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Solution

The correct option is B 23.0
Given Li (6.9), Na (X), K (39.0)
Applying the property of Dobernier's triads for the above triad-
$\frac{(6.9 + 39.0)}{2}$ $\approx$ 23.0 = X
$⟹ X = 23$

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