In 1869, Mendeleev made remarkable contribution to the classification of elements. He gave a law known as periodic law. The law states that the physical and chemical properties of elements are periodic function of their atomic weights .
On the basis of this periodic law, he arranged all the known elements in the form of table known as periodic table. It was observed that, elements with similar properties recur at regular intervals or periodically. In the Mendeleev's periodic table, horizontal rows are called periods and vertical columns are called groups. The Mendeleev's classification gave him so much confidence that he boldly left certain spaces or gaps for undiscovered elements.