Question

Which of the following is not a Döbereiner’s triads?
$[\text{Given: Atomic mass of elements}:Li=6.9,N=14,Na=23,P=31,Cl=35,K=39Ca=40,As=74.9,Br=79.9,Sr=87.6,I=126.9,Ba=137.3]$

• A. $Li,Na,K$
• B. $Ca,Sr,Ba$
• C. $N,P,As$
• D. $Cl,Br,I$

Answer 1

The correct option is C $N,P,As$

Döbereiner arranged the elements with similar properties into groups. In this group, when the three elements in a triad were written in the order of increasing atomic masses; the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.
According to the Döbereiner’s triads, avarage atomic masses of $N$ and $As$ is $\frac{14+74.9}{2}=44.45$ which is not approximately equal to atomic mass of $P$.
Hence $N,P,As$ does not form Döbereiner’s triads