Mendeleev’s Periodic Table

It was found that each element has certain peculiar properties that distinguish it from all other elements. However, if all the elements are considered together, we may see that there are certain groups that have very similar chemical properties. D. I. Mendeleev, famous Russian chemists, was the first to discover the law of dependence of the properties of elements upon their atomic weights, and in 1869 he proposed a system of classification in which the elements are arranged according to this law. The system is known as the periodic system.

The fundamental idea expressed by the arrangement of the elements in Mendeleev’s Table is that of the periodic reoccurrence of properties as the atomic weights increase.

The value of the Periodic System was demonstrated in a very striking fashion by the prediction of unknown elements made by Mendeleev. When he was constructing his table, it was obvious that several gaps should be left in order to have the succeeding elements fall into their proper groups. Such gaps were to be found in the places now filled by scandium, gallium, germanium, and from the properties of the adjacent elements and their atomic weights. The remarkable accuracy of Mendeleev’s prediction is clearly illustrated by the comparison of the unknown element called by Mendeleev “ekasilicon” with germanium discovered later by Winkler.

In the table elaborated by Mendeleev the symbols of the elements are given with their atomic weights. A study of the chart reveals that the elements are arranged in horizontal rows of ten in the order of increasing atomic weights. There are none regular groups marked “Group 0 to VII”, and the elements contained in each of these groups have similar chemical properties. The groups are further divided into (a) and (b) subgroups. The elements in group (a) have some properties in common that differentiate them from the elements in subgroup (b). However, all the elements in the vertical groups have one important chemical property in common which is known as valence.

Valence was earlier defined as that property of an atom which enables it to combine with a certain number of atoms of another element. The elements in group 0 do not have the power of combining with other elements, hence their valence is 0. The elements of group I may have valence of one, the elements of group II – valence of two, etc. Elements may have positive valence (+) or negative valence (-).