If atomic mass of Li is 7 and that of K is 39. What would be the atomic mass of Na

Li, Na and K are the elements of a

Dobereiners Triad. If atomic mass of Li is 7 and that of K is 39. What would be the atomic mass of Na? 

Answer: According of Dobereiner’s law of triads, the atomic mass of the middle element, in this case Na, should be the arithmetic mean of Li and K.

Thus, arithmetic mean of Li and K 

=  (7 + 39)/2    

= 23 (Answer)