Question

Can the following groups of elements be classified as Dobereiner’s triad:
(a) $Na,Si,Cl$
(b) $Be,Mg,Ca$
Justify the answer in each case.
[Atomic mass of $Be - 9;Na - 23;Mg - 24;Si - 28;Cl - 35;Ca - 40$ ]

Answer 1

In the history of the periodic table, Dobereiner's triads were an early attempt to sort the elements into some logical order by their physical properties. In 1817, a letter reported Johann Wolfgang Dobereiner's observations of the alkaline earths; namely, that strontium had properties that were intermediate to those of calcium and barium.

Complete step by step answer:
According to the Dobereiner law of triads, in a group of three elements or in a triplet of elements, the mean of the atomic mass of the first and the third element is equal to the atomic mass of the middle placed element. Based on this, when we compare the above two triads, we have:
(a) $Na,Si,Cl$-
The atomic mass of the silicon element is given as = $28$
The atomic mass of the silicon atom, as per the law of triads should be the mean of the atomic masses of the sodium and chlorine element.
Hence, atomic mass of silicon should be = $\dfrac{{{M_{Na}} + {M_{Cl}}}}{2} = \dfrac{{23 + 35}}{2} = \dfrac{{58}}{2} = 29$
No, because all these elements do not have similar properties although the atomic masses of silicon are average atomic masses of sodium ($Na$ ) and chlorine ($Cl$ ).
(b) $Be,Mg,Ca$-
The atomic mass of the magnesium element is given as = $24$
The atomic mass of the magnesium atom, as per the law of triads should be the mean of the atomic masses of the beryllium and calcium element.
Hence, atomic mass of silicon should be = $\dfrac{{{M_{Be}} + {M_{Ca}}}}{2} = \dfrac{{9 + 40}}{2} = \dfrac{{49}}{2} = 24.5$
Yes, because they have similar properties and the mass of magnesium ($Mg$ ) is roughly the average of the atomic mass of $Be$ and $Ca$ .
The correct option is D,hardening the metals.

Note:
By 1829, Dobereiner had found other groups of three elements (hence "triads") whose physical properties were similarly related. He also noted that some quantifiable properties of elements (e.g. atomic weight and density) in a triad followed a trend whereby the value of the middle element in the triad would be exactly or nearly predicted by taking the arithmetic mean of values for that property of the other two elements.