= Solution
The \b[large positive sum of ionization energies and large negative <hydration enthalpy> dominate], with a smaller but significant positive atomization term. Ionization depends on effective nuclear attraction, orbital energies and the stability of successive gas-ion <electron> configurations; removing the third <electron> may require disruption of a particularly stable divalent f configuration. Atomization depends on the metal's cohesive bonding and valence participation, and need not vary perfectly smoothly.
Hydration becomes more favourable for higher charge density. All the compared ions have charge $+3$, so smaller effective ionic radius generally gives more negative <hydration enthalpy>. Incomplete shielding by the 4f shell produces the <lanthanide contraction> and strengthens ion–water attraction along the series. A Born-type electrostatic estimate has a stabilization scale proportional to $-z^2/r$, but hydration-shell structure, water polarization and <temperature> dependence prevent treating that crude scale as an exact <enthalpy> formula. The hydration trend is smoother than the sharp configuration-dependent ionization anomalies. The measured formation <enthalpy> reflects cancellation of large terms, so comparison of just one term can give a misleading prediction.
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