Question

define periodicity and its cause ​

Answer 1

Periodicity : "The repetition of Certain properties by the elements at a regular interval is known as periodicity". Causes of periodicity : ... Similar elements are repeated at a regular interval When they arranged in the Increasing order of their atomic number.

Answer 2

A tabular arrangement of elements in a group and periods highlighting the regular
trends in properties of elements is called PERIODIC TABLE.
The properties that reappear at regular intervals,or in which there is gradual variation at regular intervals,are called 'periodic properties' and the phenomenon is known as the periodicity