The Correct Answer is a,d. • Atomic radius is defined as the distance from the center of the nucleus of the atom to its outermost shell. • As we move down the group, the number of shells increases in the atom, with the increase in the number of shells the atomic size increases. • As we move down the group the effective nuclear charge between the nucleus and the outermost shell starts decreasing due to which the atomic size increases. • Atomic radii of elements gradually increase in a group from top to bottom. Hence the statement (a) is correct. As we go down a group, the atomic number increases, thus, the number of shells increases. Also, as we move down the group, the valence electrons are present in a higher shell, and thus, the distance of valence electrons from the nucleus increases. As a result of this, the force of attraction between the nucleus and the valence electron decreases. Therefore, the atomic radius increases on moving down a group. • As we move across a period from left to right, the atomic radii decrease gradually. Hence the statement (d) is correct. • In a period, there are gradual increases in the nuclear charge from left to right. As the atomic number increases in a period, the electron enters the same shell. Thus, they are more and more strongly attracted to the nucleus. As a result of this force of attraction from the nucleus, the atomic radii gradually decreases.