Question

Identify a Dobereiner triad from the following.

• A. Li, Na, K
• B. N, P, As
• C. He. Ne, Ar
• D. Li, Ca, K

Answer 1

The correct option is A Li, Na, K

According to Dobereiner, when the three elements in a triad are written in the increasing order of atomic masses, the atomic mass of the middle element is roughly the average of the atomic masses of the other two elements. In the given options, only option A satisfies Dobereiner’s law.
Calculation:

Atomic masses of Li = 6.9, Na = 23.0 and K = 39.0

$⟹\frac{(6.9+39.0)}{2}=\approx 23.0$