Question

Which of the following does not belong to the group of Doberiener's triad?

• A. Calcium, strontium, and barium
• B. Chlorine, bromine, and iodine
• C. Hydrogen, lithium, and sodium
• D. Lithium, sodium, and potassium

Answer 1

The correct option is C Hydrogen, lithium, and sodium

1) Atomic masses of $\text{Ca}$ = 40, $\text{Ba}$ = 137,
$\text{Sr}$ = 88
We know that:
${\text{Atomic mass of 2}}^{\text{nd}}$ element= $\frac{{\text{Atomic mass of 1}}^{\text{st}}+{\text{Atomic mass of 3}}^{\text{rd}}}{2}$
Here, $\frac{\text{Atomic mass of Ca + Atomic mass of Ba}}{2}$
= $\frac{137+40}{2}$ =88.5 ~ 88 = the atomic mass of $\text{Sr}$
Hence, it is Dobereiner's triad.

2) Atomic masses of $\text{Cl}$ =35, $\text{I}$ =127, Atomic mass of $\text{Br}$ =80
$\frac{\text{Atomic mass of Cl + Atomic mass of I}}{2}$
= $\frac{35+127}{2}$ = 81= the atomic mass of $\text{Br}$
Hence, it is a Dobereiner's triad.

3) Atomic masses of $\text{H}$ =1 , $\text{Li}$ =6.9, $\text{Na}$ =22
$\frac{\text{Atomic wt. of H + Atomic wt of Li}}{2}\ne \text{Atomic mass of Na}$
Hence, it is not a Dobereiner's triad.

4) Atomic masses of $\text{Li}$ =6.9, $\text{Na}$ =23, $\text{K}$ =39
$\frac{\text{Atomic mass of P+ Atomic mass of Nb}}{2}\ne \text{Atomic mass of V}$
Hence, it is not a Dobereiner's triad.