What is Mendeleev's periodic law

The first real breakthrough in the classification of elements was provided by Russian chemist, Dmitri Ivanovich Mendeleev, who was the professor of chemistry at the university of Petersburg,Moscow. He has been regarded as the main contributor to the early development of the periodic table.

A periodic table may be defined as an arrangement or a chart which classifies all the known elements on the basis of their properties in such a way that the elements with similar properties are placed in the same vertical column and dissimilar elements are separated from one another.

When Mendeleev started his work on classification of elements, only 63 elements were known. He tried to correlate the atomic masses of the elements with their physical and chemical properties. Among chemical properties, Mendeleev mainly concentrated on the compounds formed by the elements with hydrogen and oxygen. He selected hydrogen and oxygen because they are highly reactive and hence formed compounds with almost all the elements. Therefore, atomic masses and similarity in the formulae of the hydrides and oxides formed by the various elements were chosen as the basis of classification of elements.

Mendeleev then took 63 cards and on each card, he wrote down the properties of one element and the formulae of its hydride and oxide. Thereafter, he sorted out the elements with similar properties and pinned their cards together on the wall one after the other in a vertical column. When he did so, he found that most of the elements automatically got arranged in order of their increasing atomic masses. He also observed that there occurs recurrence of elements with similar physical and chemical properties after certain regular intervals. On the basis of these similarities, Mendeleev proposed his periodic law. Thus, Mendeleev's Periodic Law states that :

The physical and chemical properties of elements are a periodic function of their atomic masses.

By periodic function we mean that the properties of the elements get repeated after certain regular intervals. In other words, Mendeleev's Periodic Law may also be stated as

When elements are arranged in order of their increasing atomic masses, elements with similar properties are repeated after certain regular intervals. This repetition of properties of elements after certain regular intervals is called periodicity of properties.