Give brief information on the atomic and ionic radius of alkali metals (Group $I$).

(N/A) The alkali metal atoms have the largest sizes in a particular period of the periodic table. With an increase in atomic number,the atom becomes larger.
The monovalent ions $(M^{+})$ are smaller than the parent atom because the number of electrons decreases while the nuclear charge remains the same,leading to greater effective nuclear attraction.
The atomic and ionic radii of alkali metals increase on moving down the group,i.e.,they increase in size while going from $Li$ to $Cs$.