Answer :
Atomic radii from the left to right across a period tend to decrease.
Atomic radii from the top to the bottom of a group tend to increase.
Explanation :
Atomic radius of an atom is defined as the total distance from the nucleus to the valence shell of the atom.
As moving from left to right in a period, more electrons are added in the same shell and the attraction between the last electron and nucleus increases, which results in the shrinkage of size of an atom.
Thus, atomic radii from the left to right across a period tend to decrease.
As moving from top to bottom in a group, there is an addition of shell around the nucleus and the outermost shell gets far away from the nucleus and hence, the distance between the nucleus and outermost shell increases.
Thus, atomic radii from the top to the bottom of a group tend to increase.
