Mendeleef’s Classification of Elements

Dmitri I. Mendeleef (1834-1907) was in his thirties (1869) when he developed the periodic table. While working on classification he found that when elements are arranged in order of increasing atomic masses, the elements with similar properties occur at regular intervals. Based on these observations, Mendeleef gave a periodic law in 1869. According to Mendleef’s periodic law, “The properties of elements are periodic function of their atomic masses.”

On the basis of this periodic law, Mendeleef constructed a periodic table in such a way that the elements were arranged horizontally in order of their increasing atomic masses. However, he also kept in mind the similarities in the chemical properties of the elements. The main criterion of the judgement of similarities in the properties was valency of elements and their formula’s of oxides and hydrides formed.

Mendeleef’s modified periodic table consists of: 

Atomic numbers of elements are given on the left hand upper corner of their symbols while atomic masses are given under their symbols.

(I).   Nine Vertical Column’s called Groups, these are numbered from I to VIII and zero (The zero group members were not known then, at that time only 63 elements were known) . Each group from I to VII, is further sub-divided into two sub- groups designated as ‘A’ and ‘B’. Group VIII contains three sub-groups. Group zero consists of inert gases.

(II).  Seven Horizontal Rows called Periods. These are numbered from 1 to 7. First period contains two elements. Second and third period contain eight elements each. These periods are called short periods. Fourth and Fifth periods contain eighteen elements each. These periods are called Long Periods. Sixth period contains 32 elements and is called Longest Period, Seventh period is incomplete and contains nineteen elements.