Cause of Periodicity

It has been observed that when elements are arranged in order of increasing atomic numbers, the elements with similar properties are repeated after regular intervals.

This periodical repetition of elements with similar properties after certain regular intervals when elements are arranged in order of increasing atomic number is called PERIODICITY.

Since the electrons present in the inner shells do not take part in chemical reaction, it must be the electrons in the outer most shell which control the properties of the atoms. Thus, if the arrangement of electrons in the outer shell or valence shell are same, the properties will also be similar. It is found to be true, if we examine the electronic configurations of Alkali metals elements, it is observed that each elements has only one electron in their valence shell i.e. the electronic arrangement in these elements are similar and this arrangement is repeated after certain regular intervals with the regular increase of atomic number. Thus, it can be concluded that the Periodic repetition of properties is due to the repetition of similar valence shell electron configuration after certain regular intervals.

It may be noted that this type of similarity in properties is repeated after intervals of 2, 8, 8, 18, 18, 32 in their atomic numbers. These numbers are known as Magic Numbers.