Question

what is meant by periodicity of properties of elements? why are properties of elements placed in the same group of the periodic table similar?

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.