Question

Can the following groups of elements be classified as Dobereiner's triad?

(a) Na, Si, Cl

(b) Be, Mg, Ca

The atomic mass of Be-9, Na-23, Mg-24, Si-28, Cl-35, and Ca-40 explain by giving a reason.

Answer 1

Earlier many attempts were made to classify the elements into different groups so that their studies become convenient and easy. In order, the German scientist Johann Wolfgang Dobereiner also gave a law known as triad law.

Dobereiner's triad: If we arrange3 three elements that have the same physical and chemical properties in their increasing order of atomic weight the arithmetic mean of first and last will be the atomic weight of the middle element.

The common triad of the Doberiener classification were Lithium(Li), Sodium(Na), Potassium(K) ; calcium(Ca), Strontium(Sr), Barium(Ba) ; Chlorine(Cl), Bromine(Br), Iodine(I) etc.

(a) In this case as we have already given the atomic weight of different elements

Na: 23

Si 28

Cl: 35

According to triad law:

The atomic weight of Si : $\frac{23+35}{2}$

= $\frac{58}{2}$

= $29$

Here, the average atomic mass is true but the physical and chemical properties are not the same in these three elements that's why they can not be a triad.

(b) In this case as we have already given the atomic weight of different elements

Be: 9

Mg: 24

Ca: 40

According to triad law:

The atomic weight of Si : $\frac{9+40}{2}$

= $\frac{49}{2}$

= $24.5$

In this case, Mg mass is also the average of both the elements and the physical and chemical properties are also the same so yes they are a triad.