Question

The set of do not form a Döbereiner’s triad.

• A. $N\left(14\right),P\left(31\right),\mathrm{As}\left(75\right)$
• B. $\mathrm{Ca}\left(40\right),\mathrm{Sr}\left(88\right),\mathrm{Ba}\left(137\right)$
• C. $\mathrm{Cl}(35.5),\mathrm{Br}\left(80\right),I\left(127\right)$

Answer 1

The correct option is A $N\left(14\right),P\left(31\right),\mathrm{As}\left(75\right)$

According to Döbereiner, 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.

Cl, Br, I: $\frac{(35.5+127)}{2}$ $\approx$ $80$ amu which is the atomic mass of Br. Thus, Cl, Br, and I form Döbereiner’s triad.

Ca, Sr, Ba: $\frac{(40+137)}{2}$ $\approx$ $88$ amu which is the atomic mass of Sr. Thus, Ca, Sr, and Ba form Döbereiner’s triad.

N, P, As: $\frac{(14+75)}{2}$ $\approx$ $44.5$, which is not the atomic mass of P.

Hence, N, P, and As do not form Döbereiner’s triad.