Question

Which of the following elements follow Dobereiner’s law of triads?
A.${\rm{Na}} = 23$, ${\rm{Li}} = 7$and ${\rm{K}} = 39$
B.${\rm{Cl}} = 35$, ${\rm{Br}} = 80$and ${\rm{F}} = {\rm{19}}$
C.${\rm{Ca}} = 40$ , ${\rm{Mg}} = 24$and ${\rm{Sr}} = 88$
D. ${\rm{Na}} = 23$, ${\rm{K}} = 3{\rm{9}}$and ${\rm{Rb}} = 85$

Answer 1

Hint:
Dobereiner’s triad was one the early attempts made to group elements on the basis of their properties.According to this law of triads, the arithmetic mean of the first and third element of the triad is equal to the atomic mass of the second element.

Complete step by step answer:
First let's discuss Dobereiner triads. Law of triads was given by a German scientist whose name is Johann Wolfgang Dobereiner. He observed that the group of three elements could be formed in where all the elements possess similar physical and chemical properties. The triads identified by him was, Lithium, sodium, potassium (triad 1), calcium, strontium, barium (triad 2), chlorine, bromine, iodine (triad 3), sulphur, selenium, tellurium (triad 4) and iron, cobalt, nickel (triad 5) .
Now, we will check which option follows Dobereiner’s law of triads or not. For this we have to calculate the arithmetic mean of the first and third element. If the result is the atomic mass of the second element, then the elements follow Dobereiner’s law of triads.
${\rm{Li}} = 7$,${\rm{Na}} = 23$ and ${\rm{K}} = 39$
$\begin{array}{c}{\rm{Arithmetic}}\;{\rm{mean}} = \dfrac{{7 + 39}}{2}\\ = 23\end{array}$
So, the result is 23 which is the atomic mass of the second element, that is, sodium. So, they form triads. Hence, option A is correct.
Similarly we calculate the arithmetic mean for the elements in option B.
${\rm{F}} = {\rm{19}}$${\rm{Cl}} = 35$ and ${\rm{Br}} = 80$
$\begin{array}{c}{\rm{Arithmetic}}\;{\rm{mean}} = \dfrac{{80 + 19}}{2}\\ = 49.5\end{array}$
So, the result is 49.5 which is not the atomic mass of chlorine. So, they do not form triads. Hence, option B is not correct.
Similarly we calculate the arithmetic mean for the elements in option C.
${\rm{Mg}} = 24$,${\rm{Ca}} = 40$and ${\rm{Sr}} = 88$
$\begin{array}{c}{\rm{Arithmetic}}\;{\rm{mean}} = \dfrac{{24 + 88}}{2}\\ = 56\end{array}$
So, the result is 56 which is not the atomic mass of calcium. So, they do not form triads. Hence, option C is not correct.
Similarly we calculate the arithmetic mean for the elements in option D.
${\rm{Na}} = 23$, ${\rm{K}} = 3{\rm{9}}$and ${\rm{Rb}} = 85$
$\begin{array}{c}{\rm{Arithmetic}}\;{\rm{mean}} = \dfrac{{23 + 85}}{2}\\ = 54\end{array}$
So, the result is 54 which is not the atomic mass of potassium. So, they do not form triads. Hence, option D is not correct.

So the correct option is A.

Note:
Doberier’s triads and Newlands law of octaves were the early attempts made to group elements on the basis of their properties. According to Newland’s law of octaves, every eight elements possess similar properties on arranging elements in order of increasing atomic mass.