Arrange the following elements in the increasing order of their atomic radii:
$(i)$ $O, N, F, B, Be, Li, C$
$(ii)$ $P, S, Mg, Na, Al, Si, Cl$
$(iii)$ $K, Na, Li, Cs, Rb$
$(iv)$ $Cl, F, I, At, Br$

(A) Atomic radius decreases across a period from left to right and increases down a group.
$(i)$ These elements belong to the $2^{nd}$ period. The order of increasing atomic radius is: $F < O < N < C < B < Be < Li$.
$(ii)$ These elements belong to the $3^{rd}$ period. The order of increasing atomic radius is: $Cl < S < P < Si < Al < Mg < Na$.
$(iii)$ These elements belong to Group $1$. The order of increasing atomic radius is: $Li < Na < K < Rb < Cs$.
$(iv)$ These elements belong to Group $17$. The order of increasing atomic radius is: $F < Cl < Br < I < At$.