Henry Moseley, an English physicist discovered in the year 1913 that atomic number, is the most fundamental property of an element and not its atomic mass. Atomic number, (Z), of an element is the number of protons in the nucleus of its atom. The number of electrons in the neutral atom is also equal to its atomic number. This discovery changed the whole perspective about elements and their properties to such an extent that a need was felt to change the periodic law also. Now we shall learn about the changes made in the periodic law.
4.2.1 Modern periodic law
After discovery of atomic number the periodic law was modified and the new law was based upon atomic numbers in place of atomic masses of elements.

The Modern Periodic Law states"The chemical and physical properties of elements are a periodic function of their atomic numbers"

After the change in the periodic law many changes were suggested in the periodic table. Now we shall learn about the modern periodic table which finally emerged.