In the eighteenth century many chemists began to identify elements and study their properties. A lot of attempts were made to classify them. The most outstanding classification was made by the Russian chemist Dmitry Mendeleev, who published his Periodic Table of Elements in 1869. Mendeleev noticed that if he arranged the elements in order of increasing atomic weight, definite chemical properties repeated themselves periodically. His table of the known elements was constructed with seven elements in each of the first two rows and seventeen elements in each of the next two rows. But there were several blank spots in his table of those elements, which had not been discovered at that time. Mendeleev predicted their existence basing on the observation of periodicity. He even called them ekaboron, ekaaluminum and ekasilicon. Finally, the discovery of gallium, scandium and germanium were made. Mendeleev's table revealed the fact that properties of the elements are periodic function of their atomic masses. This statement was called the Periodic Law. 
Now the Periodic Law reads as follows: the physical and chemical properties of the elements are periodic functions of their atomic numbers. The elements are arranged in order of increasing atomic numbers. The value of the Periodic Table consists in the fact that chemists can predict the properties of their compounds by grouping the elements into families and knowing the trends within these families.
