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F < N < Be < Li (Li, Be, F and N belong to same period of modern periodic table. As we move from left to right in a group, the atomic radii of elements decreases due to high atomic charge but the number of shell remain constant).

(b) Cl < Br < I < At (Cl, At, Br and I belong to same group of periodic table that is group 17. As we move down in a group, the atomic radii of elements increase due to increase in the number of shell of an element).