Question

Why physical and chemical property are periodic function of their atomic number

Answer 1

it is because atomic no. helps to know the physical
properties of elements in a more precise manner than atomic mass.

Answer 2

in the  periodic table the the properties of elements appear after regular intervals. for eg. in a group the valency remains the same and thus the their properties re same.so as, the electronic configuration recurs after intervals. thus when the elements are arranged as per atomic no. they show similar function and properties . 

hope it helps!!