What is lanthanoid contraction? Give its cause and consequences. Compare the lanthanoids and actinoids with respect to configuration, oxidation states and chemical reactivity.

1. 1 · Define in one line. Lanthanoid contraction is the steady decrease in atomic and ionic radii from La to Lu.
2. 2 · Give the cause. Imperfect shielding of one 4f electron by another as the nuclear charge rises.
3. 3 · Give the consequences.
   • Zr (160 pm) and Hf (159 pm) are almost the same size.
   • Lanthanoids are hard to separate.
   • Basicity of Ln(OH)₃ decreases from La to Lu.
4. 4 · Compare configuration. Lanthanoids [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s²; actinoids [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s².
5. 5 · Compare oxidation states. Lanthanoids mainly +3; actinoids +3 to +7, because 5f, 6d and 7s have similar energies.
6. 6 · Compare reactivity. Both reactive metals; actinoids are all radioactive and highly reactive when finely divided.
7. 7 · Link to real life. Uranium and thorium (from Kerala's monazite sands) are nuclear fuels.

Scoring tip: Quote the Zr 160 pm and Hf 159 pm values as your first consequence; examiners look for this pair.