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:

The first attempt towards the classification of elements was made by John Dobereiner a German chemist in 1829. He classifies 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:

The English chemist John Newland in 1865 − 66, developed the law of octaves. He 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 1869:

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 similar positions on the graph. Strong electro-positive elements of the IA group except for 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.

Mendeleev’s Periodic Law:

In 1869 Dmitri Mendeleev, a Russian Chemist used broader range to physical and chemical properties to classify the elements. The physical and chemical properties of elements are periodic functions of their atomic weights. If the elements are arranged in the order of their increasing atomic weights, after a regular interval elements with similar properties are repeated (inert gases were not discovered then)