Question

Identify the option that is not one of the Dobereiner's triad?

• A. Chlorine-35, Bromine-80, Iodine-127
• B. Potassium-39, Chlorine-35.5, Bromine-80
• C. Calcium-40, Strontium-88, Barium-137
• D. Lithium-6, Sodium-22, Potassium-39

Answer 1

The correct option is B Potassium-39, Chlorine-35.5, Bromine-80

A Dobereiner's triad is formed in such a way that the average atomic mass taken of the first and last element is equal to the atomic mass of the element in the middle. On applying this to the given options we have:

1) Calcium (Ca)-40, strontium (Sr)-88, barium (Ba)-137
$\frac{\text{Atomic mass of Ca + Atomic mass of Ba}}{2}=\text{Atomic mass of Sr}$
Arithmetic mean of first and third element=$\frac{\text{40+137}}{2}=\text{Atomic mass of Sr}=88$
Hence it is a Dobereiner's triad.

2) Chlorine (Cl)-35, bromine (Br)-80, iodine (I)-127
$\frac{\text{Atomic mass of Cl + Atomic mass of I}}{2}=\text{Atomic mass of Br}$
Arithmetic mean of first and third element=$\frac{\text{35+127}}{2}=\text{Atomic mass of Br}=80$
Hence it is a Dobereiner's triad.

3) Lithium (Li)-6, sodium (Na)-22, potassium (K)-39
$\frac{\text{Atomic mass of Li+ Atomic mass of K}}{2}=\text{Atomic mass of Na}$

Arithmetic mean of first and third element=$\frac{\text{6+39}}{2}=\text{Atomic mass of Na}=22$
Hence it is a Dobereiner's triad.

4) Potassium (K)-39, chlorine (Cl)-35.5, bromine(Br)-80
$\frac{\text{Atomic mass of K+ Atomic mass of Br}}{2}\ne \text{Atomic mass of Cl}$
Hence it is not a Dobereiner's triad.