Atomic radii increases, as the number of shells increases. Thus, on moving down a group atomic radii increases. The electronic configuration of the given element is Mg12 = [Ne] 3s2 Ca20 = [Ar] 4s2 P15 = [Ne] 3s23p3 Cl17 = [Ne] 3s23p5 On the other hand, on increasing the number of electron in the same shell, the atomic radii decreases because effective nuclear charge is increases. In Mg, P and Cl, the number of electrons are increasing in the same shell, thus the order of their atomic radii is Cl < P < Mg In case of Ca, the electron is entering in higher shell. So, its atomic radii is highest. Thus, the order of radii is Cl < P < Mg < Ca