Anomalous Atomic Radius & Ionization Enthalpy Trends

Atomic Radius Order (Anomalous):

$$\text{Atomic Radius: } \text{B} < \text{Ga} < \text{Al} < \text{In} < \text{Tl}$$

Ionization Enthalpy ($$\Delta_i H_1$$) Order (Anomalous):

$$\text{Ionization Enthalpy: } \text{B} > \text{Tl} > \text{Ga} > \text{Al} > \text{In}$$

Concept: Ga is unexpectedly smaller than Al ($$\text{Ga} < \text{Al}$$) due to poor shielding by 10 filled 3d electrons (transition contraction), leading to a higher effective nuclear charge ($$Z_{\text{eff}}$$) on Ga. The high value of Tl is due to poor shielding by 14 filled 4f electrons (lanthanoid contraction).