State one example of a Dobereiner's traid, showing in it that the atomic mass of middle element is half-way between those of the other two.
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 an atomic mass of $$23$$.
According to Dobereiner,
$$Atomic\ mass\ of\ sodium=$$ $$ \dfrac{Atomic\ mass \ of Li+ Atomic\ mass\ of \ K}{2} $$ = $$ \dfrac{7+39}{2} $$ = $$23$$
This indicates that sodium is half way between lithium and potassium.