Question

what is atomic radius?how does the atomic radii of the element vary:

Answer 1

the atomic radius increase when we go from up to down in the modern periodic table due to addition of new subshell and decrease from left to right due to increase in no. of electrons increase in same sub shell
defination-the average distance from the center of the atom to the outermost boundary :)

Answer 2

the distance between the center of the atom and the valence shell is called atomic radius.
across the period the atomic radius decreases and down the group atomic radius increases.