Though Mendeleev’s periodic table included all the elements, yet at many places a heavier element had to be placed before a lighter one. Such pairs of elements (called anomalous pairs) violated the periodic law. Also, there was no place for different isotopes of an element in the periodic table.
Due to these reasons, it was felt that the arrangement of elements in the periodic table should be based on some other property which is more fundamental than the atomic mass.
In 1913, Henry Moseley, an English physicist discovered that the atomic number and not the atomic mass is the most fundamental property of an element. Atomic number (Z) of an element is the number of protons in the nucleus of its atom.
Since atom is as electrically neutral entity, the number of electrons is also equal to its atomic number i.e. the number of protons. After this development, it was felt necessary to change the periodic law and modify the periodic table.
The Modern Periodic Law states that the chemical and physical properties of elements are periodic functions of their atomic numbers. If elements are arranged in the order of their increasing atomic number, the elements with similar properties are repeated after certain regular intervals.
Fortunately, even with the revised periodic law, the Mendeleev’s classification did not require any major revision as it was based on properties of the elements. In fact, taking atomic number as the basis for classification, removed major defects from it such as anomalous pairs and position of isotopes. After changes in the periodic law, many modifications were suggested in the periodic table.
Consider the electronic configuration of alkali metals i.e., the first group elements with atomic numbers 3, 11, 19, 37, 55 and 87 (lithium, sodium, potassium, rubidium, caesium and francium). All these elements have one electron in the outer most shell and so they have similar properties which are:
All the elements having similar electronic configuration have similar properties. Thus, the re-occurrence of similar electronic configuration is the cause of periodicity in properties of elements.