State one example of a Dobereiner's triad, showing in it that the atomic mass of middle element is half-way between those of the other two.

Lithiium, sodium and potassium are said to be triads in the Mendeleev’s periodic table. Lithium is the first element in the triad and sodium being middle has atomic mass of 23 and third element is potassium.

Atomic mass of sodium = At. Mass of Li + At. Mass of K / 2

= 7+39/2

=23