HISTORY OF DEVELOPMENT OF THE PERIODIC TABLE

All earlier attempts on the classification of the elements were based upon their atomic weights.

Dobereiner’s Triads:

Dobereiner classified the elements with similar properties in the groups of three elements (Triads). He could succeed in making only a few triads. In the triads of elements the atomic weight of the middle element was the arithmetic mean of the atomic weights of the other two. Some of the triads are as under.

Li Na K Ca Sr Ba
7 23 39 40 88 137
P As Sb Cl Br I
31 75 120 35.5 80 123

 

The major drawback of Dobereiner’s classification was that the concept of triads could be applied only for the limited number of elements.

Newland’s Law of octaves:

Newland pointed out that, if the elements are arranged in order of their increasing atomic weights, every eighth element had similar properties to first one like the first and eighth note of the musical scale. For example

sa re ga ma pa dha ni sa
Li Be B C N O F Na
Na Mg Al Si P S Cl K

 

This generalization was also discarded since it could not be applied to elements having atomic weight greater than that of calcium i.e. 40 a.m.u Furthermore with the discovery of noble gas, the properties of the eighth elements were no longer similar to the first one.

Lothar Meyer’s Atomic Volume Curve:

Lothar Meyer a German chemist plotted a graph between atomic weight and atomic volume (i.e. atomic weight in solid state/density) and he find out that elements with similar properties occupied the similar positions on the graph. Strong electropositive elements of IA group except Li i.e. Na, K, Rb, Cs etc. occupied the top position on the graph. IIa group elements like Be, Mg, Ca, Sr, Ba etc. occupied the positions on the ascending part of the graph. Inert gases except He occupied the positions on the descending part of the graph. Halogens also occupied the descending part of the graph.

CBSE Class 12 Chemistry Notes

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