Mendeleev’s Periodic Law states that the properties of the elements are a periodic function of their atomic weights. Mendeleev arranged the elements in order of their increasing atomic weights so that elements with similar properties fell into the same vertical column.

In contrast, the Modern Periodic Law states that the physical and chemical properties of the elements are periodic functions of their atomic numbers. This approach is based on the discovery by Henry Moseley that atomic number is a more fundamental property of an element than its atomic mass. Thus, the basic difference lies in the basis of classification: atomic weight versus atomic number.