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In the Mendeleev’s classification of elements, he arranged all 63 elements in the order of their increasing relative atomic masses in the form of a table. It is known as Mendeleev’s Periodic Table. This table was divided into 8 columns and 7 rows. The columns are known as groups and rows are known as periods. Groups from 1 to 7 comprise normal elements and group 8 comprised a few transition elements. Elements with similar properties had been kept in the same group. For example; lithium, potassium, rubidium are kept in 1st group. These elements usually formed compounds such as oxides and hydrides. Such properties formed the basis of classification of these elements. For example, hydrogen, sodium, and potassium belong to the first group. The general formula of oxides for the elements of 1st group is R₂O (H₂O, Na₂O, K₂O).
He arranged all the known elements in increasing order of their atomic masses. Elements with similar properties fall in same group. However, Mendeleev placed many elements in wrong order of their increasing atomic masses. For example, the atomic mass of nickel is less than that of cobalt but still cobalt was placed before nickel.
Mendeleev left some blank spaces intentionally in his periodic table in order to place the elements having similar properties in the same group in future. For example, titanium has been placed in 4th group, leaving a blank space adjacent to it in 3rd group. Similarly, arsenic has been placed in 5th group; leaving two adjacent spaces blank. These spaces have been occupied by scandium, gallium and germanium after their subsequent discoveries.
Mendeleev discovered some elements and named them as eka-boron, eka-aluminium and eka-silicon. Scandium, Gallium and Germanium were discovered later and took the place of eka-boron, eka-aluminium and eka-silicon, respectively in the gap left in the Mendeleev’s Periodic table. Their properties were exactly similar to the corresponding predicted elements.

Thus, Mendeleev’s Periodic Law states that the properties of elements are the periodic function of their relative atomic masses. Noble gases, being inert, could be placed in a separate group without disturbing the original order in the Mendeleev’s Periodic Table.