Question

Lithium, sodium and potassium form a Dobereiner’s triad. The atomic masses of lithium and potassium are 7 and 39 respectively. Predict the atomic mass of sodium. [2 MARKS]

Answer 1

Law of Triads: According to Dobereiner, when elements are arranged in increasing order of their atomic masses, the arithmetic mean of the atomic masses of the first and third element in a triad is approximately equal to the atomic mass of the second element in that triad.

The atomic masses of lithium and potassium are 7 and 39 respectively.

Atomic mass of sodium = $\frac{\text{Atomic mass of lithium + Atomic mass of potassium}}{\text{2}}$ $\Rightarrow$ $\frac{\text{7+39}}{\text{2}}$
$\Rightarrow$ 23.