Question

What is periodicity in properties of elements with refrence tothe modern periodic table? Why do all the elements of same group have similar properties ? How does the tendancy of elementsto gain electron change as we move from left to right in a period ? State the reason for this change

Answer 1

The occurrence (or arrangement) of the elements with similar properties after certain regular interval when they are arranged in increasing order of atomic number is called periodicity. The periodic repetition of the properties is due to the recurrence of similar valence shell configuration after certain regular interval.  

So, because the elements in a group have similar electronic configuration, they have same number of valence electrons and therefore they show similar chemical properties. 

Hope this helps!