63.

Discuss the earlier attempts to classify elements.

Different scientists tried to group elements with similar elements.
(i) Dobereiner Triads (1822). Dobereiner arranged similar elements in a group of three so that the atomic mass of the central element was nearly equal to arithmetic mean of the atomic mass of the other two elements. The group of three elements was named as triad. For example,
Triad                    Lithium     Sodium          Potassium           Mean atomic mass
Atomic masses       7                23                    39                     
Triad                  Chlorine        Bromine              Iodine             Mean atomic mass
Atomic masses       35.5            80                      127                    
But this concept of triad could be applied to a limited number of elements. It was rejected soon.

(ii) Newland's law of octaves (1864). Newland observed that if the known elements were arranged in order of their increasing atomic masses, similar properties recurred in every eighth element like nodes in a musical scale. This generalisation was known as Law of octaves. This system worked very well for lighter elements.

 

But this law failed in case of heavier elements and was rejected.

(iii) Lother Meyer’s work (1870). Lother Meyer plotted a graph between atomic volumes of elements in the solid state and their atomic masses. He found that the elements with similar physical properties occupied similar positions on the curve. By taking into account this fact, he drew up a periodic table in which elements were arranged in order of increasing atomic masses.