Question

Given below are the elements with their atomic masses. Identify a set of elements which is not a Dobereiner's triad.

• A. Li (7), Na (23.0), K (39)
• B. N (14.0), P (31.0), As (75)
• C. Ca (40), Sr (88), Ba (137)
• D. Cl (35.5), Br (80), I (127)

Answer 1

The correct option is B

N (14.0), P (31.0), As (75)

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. On doing the calculation for the atomic masses of the given elements we get:

Lithium, sodium, potassium: $\frac{(7+39.0)}{2}$ $\approx$ $23.0$
Nitrogen, phosphorus, arsenic: $\frac{(14+75)}{2}$ $\approx$ $44.5$ but not $31.0$
Calcium, strontium, barium: $\frac{(40+137)}{2}$ $\approx$ $88$
Chlorine , bromine, iodine: $\frac{(35.5+127)}{2}$ $\approx$ $80$

So, nitrogen ($\text{N}$), phosphorous ($\text{P}$) and arsenic ($\text{As}$) are elements which are not satisfying Dobereiner's triad.