define periodicity and give its cause?
The trend of similar properties that are observed in elements, when arranged or positioned with increasing order of atomic number called periodicity.
The cause for the periodicity is the valence electrons of elements. Valence electrons influence the properties of elements. When elements are arranged in the order of increasing atomic number, then elements with same number of valence electrons are grouped together. Members of same group have same electronic configuration and hence same valency. Thus, members of a group show similar chemical properties, because of which they show periodicity.
The periodic repetition of the elements with similar properties after certain regular interval when atoms are arranged in increasing order of atomic number is called periodicity.
The periodic repetition of the properties is due to the recurrence of the similar valence shell configuration after certain regular interval.