Question

What's the rationale for periodicity in properties?

Answer 1

Hint : Within the context of chemistry and therefore the table, periodicity refers to trends or recurring variations in element properties with increasing numbers. Periodicity is caused by regular and predictable variations in element atomic structure.

Complete Step By Step Answer:
Repetition of properties after a particular interval is named periodicity of properties. If elements are arranged in increasing order of their number within the table, then elements repeat their properties after a particular interval.
The explanation for periodicity of properties of elements is thanks to the repetition of comparable electronic configuration of their atoms within the outermost energy shell after a certain regular interval.
Periodicity is the fact of something happening at regularly-spaced periods of your time. An example of periodicity is that the full-of-the-moon happens every 29.5 days. The tendency, quality, or the very fact of recurring at the regular time of intervals. The occurrence of comparable properties in elements occupying similar positions within the table.
According to Mendeleev's law, the properties of the element and their compounds are a periodic function of their atomic numbers. When elements are placed consistent with the number, then similar electronic configurations are repeated after regular intervals.

Note :
The playing period contains only eight elements albeit the electron capacity of the third shell is eighteen because when the opposite shells get filled and therefore the resultant number of electrons becomes eighteen, it gets added up and settles within the third group and three shells are acquired by the fourth period.