Knowledge of the structure of the atom led to the discovery of a new property of the elements called the atomic number. The atomic number of an element is equal to the number of protons in its atom. It is also equal to the number of electrons present in the neutral atom. No two elements can have the same value as an atomic number. The isotopes of an element have the same atomic number but different atomic weights. Because of the same atomic number, they possess similar chemical properties. These facts indicate that the atomic number is a more fundamental property than the atomic weight and hence it can form a better basis of classification of elements. With the discovery of atomic number as the fundamental property of elements, Mendeleev’s periodic law was modified. Moseley stated the modern periodic law as under-
The properties of elements are periodic functions of their atomic numbers.
With the change in the basis of classification from atomic weight to atomic number, the following defects have disappeared.
Position of Isotopes- The atomic numbers of the isotopes of an element are same and hence they do not require separate places.
Existence of Anomalous Paris of Elements- The arrangement of elements in the order of their increasing atomic numbers make them occupy proper places and thus the defect, regarding the existence of anomalous pairs of elements due to some elements having higher atomic weights preceding those having lower atomic weights, disappears.