- Atomic radius decreases across the period. Hence, the atomic radius of Mg is larger than that of Al.
- Parent atoms have larger radius than their corresponding cations. Hence, the radius of Mg is larger than that of Mg2+ and the radius of Al is larger than that of Al3+.
- Mg2+ and Al3+ are isoelectronic. Among isoelectronic species, the one with larger nuclear charge will have smaller radius. Al3+ (Z = 13) has a larger nuclear charge than that of Mg2+ (Z = 12). Hence, the ionic radius of Al3+ is smaller than Mg2+.
- Therefore, the decreasing order of radius is Mg > Al > Mg2+ > Al3+.
Hence, species with the largest size is Mg and with the smallest size is Al3+.