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.

Answer 1

According to Dobereiner's theory for triads, the atomic mass of the middle element of the triad is approximately equal to the average of atomic masses of other two elements.
That says,
Atomic mass of Lithium = 3 u
Atomic mass of Potassium = 39 u

$\begin{array}{ccc}\mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{Na}& =& \frac{7+39}{2}\\ & =& \frac{46}{2}\\ & =& 23u\end{array}$