What is the modern periodic law ?

In 1913, H.G.J Moseley in England proved that the more fundamental properties of an element are its atomic number. Therefore he suggested that the basis of classification of elements should be atomic number. This led to the modern periodic law, which states that,

“The properties of elements are periodic function of their atomic number.”

How did atomic number prove mass better basis of classification

By choosing atomic number as the basis of classification, the anomalies in the original Mendeef’s periodic table got removed, as described below.

Inversion in Mendeleef’s Table:

The anomaly of having the element with higher atomic mass element before the element with lower atomic mass is removed because the element having higher atomic mass had the lower atomic number in some cases. For example, the atomic number of Ar, is 18 and that of K is 19, although the atomic mass of Ar is higher than that of K.

Position of isotopes:

Although isotopes of an element have different atomic masses, but they have the same atomic number. Hence, according to the atomic number as the basis of classifications, these should be allotted the same place in the table.