Question

The law of triad is applicable to a group of :

• A. $Cl,Br,I$
• B. $Ca,Sr,Ba$
• C. $Na,K,Rb$
• D. $H,O,N$

Answer 1

Answer: (A), (B)

The correct options are
A $Ca,Sr,Ba$
B $Cl,Br,I$

According to Dobereiner law of triads, the atomic weight of a middle element is almost the average of the other two. Thus, the law of triad applies to (a) $Cl,Br,I$ and (b) $Ca,Sr,Ba$. However, it does not apply to (c) $Na,K,Rb$ and (d) $H,O,N$.

(a) For $Cl,Br$ and $I,$$\frac{35.5+126.9}{2}=81.2\simeq 80u$

(b) For $Ca,Sr,Ba,$ $\frac{40.1+137.3}{2}=88.7u\simeq 87.6u$

This is not observed in the case of $Na,K,Rb$ and $H,O,N$.

(c) For $Na,K,Rb,$ $\frac{23+85.4}{2}=54.2u$

(d) For $H,O,N,$ $\frac{1+14}{2}=7.5u$