As the 14 lanthanoid elements (Ce to Lu) are filled, the 4f electrons provide very poor shielding of nuclear charge (f-orbitals are diffuse and have poor penetration). The steady increase in Zeff causes a gradual decrease in atomic/ionic radius across the series: $La^{3+}$ (103 pm) → $Lu^{3+}$ (86 pm). Consequences: (1) Post-lanthanoid elements (Hf, Ta, W, Re, Os, Ir, Pt, Au) have nearly the same radii as their 4d counterparts (Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag). Hf (159 pm) ≈ Zr (160 pm). (2) Separation of lanthanoids is difficult (similar radii, similar chemistry). (3) 5d transition metals are denser than expected (small radius, high mass).