Question

why are physical and chemical properties periodic function of there atomic number

Answer 1

this is based on Henry Moseley , experiment .
when Henry Moseley plotted a graph of square root of the frequency of certain characteristic X-ray versus atomic number . he got a straight line . but the same is not observed for a graph of √frequency versus atomic mass .
from this experiment it was confirmed that , physical and chemical properties periodic function of their atomic number not atomic mass .

in other way ,
if we arranged in the order of the increasing order of their atomic number , we see after regular interval , the periodic properties (physical and chemical properties of elements ) are repeated accurately.
if we choose periodic property based on atomic mass then some dimerit comes for arranging elements .so, we choose atomic number in place of atomic mass .