i. In 1913, Henry Moseley, an English physicist, found that the atomic number (Z) is the most fundamental property of an element and not its atomic mass. So, atomic number must be used for arranging elements in the periodic table.
ii. Atomic number (Z) of an element is the number of protons in the nucleus of the atom. It also represents the number of electrons present in the outer shells.
iii. This discovery changed the whole perspective about elements and their properties.
iv. Accordingly, Mendeleev’s periodic law was modified into Modern Periodic Law, which states that ‘the chemical and physical properties of elements are a periodic function of their atomic numbers’. The periodic table based on modern periodic law is called the modern periodic table.
