Question

In Dobereiner's triad containing $Li,Na,K$ if atomic masses of Lithium & Potassium are $6.9$ and $39.1$, then what will be the atomic mass of Sodium?

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.

Hence, the atomic mass of sodium is the arithmetic mean of the atomic masses of the first and third element in a triad.

Atomic mass of sodium $=\frac{6.9+39.1}{2}=\frac{46}{2}=23$