Structure & Physical Properties

1. All actinoid metals are silvery in appearance.
2. They show greater structural variability than lanthanoids.
3. Their metallic radii vary irregularly, much more than in lanthanoids.
4. They are generally dense metals with high melting points.

Chemical Reactivity

5. Actinoids are highly reactive metals, especially in finely divided form.
6. They react with boiling water to form a mixture of oxide and hydride.
7. They combine readily with most non-metals at moderate temperatures.
8. Hydrochloric acid (HCl) attacks all actinoid metals.
9. Most actinoids are only slightly affected by nitric acid (HNO₃) due to formation of a protective oxide layer.
10. Alkalies (NaOH, KOH) generally have no action on actinoids.

Magnetic Properties

11. Magnetic properties of actinoids are more complex than those of lanthanoids.
12. Magnetic susceptibility generally increases with the number of unpaired 5f electrons.
13. For comparable electronic configurations, lanthanoids show higher magnetic susceptibility than actinoids.

Ionisation Enthalpy & Bonding

14. Early actinoids have lower ionisation enthalpies than corresponding lanthanoids.
15. 5f orbitals are less penetrating than 4f orbitals.
16. 5f electrons are more effectively shielded from nuclear charge.
17. Outer electrons are less firmly held and participate more readily in bonding.
18. Therefore, actinoids show greater covalent character and variable oxidation states.

Comparison with Lanthanoids

19. Early actinoids differ significantly from lanthanoids.
20. Similarity with lanthanoids becomes more evident in the second half of the actinoid series.
21. Like lanthanoids, actinoids show close similarities among themselves.
22. Their properties change gradually across the series.

Actinoid Contraction

23. Actinoids exhibit actinoid contraction (decrease in atomic/ionic radii across the series).
24. Actinoid contraction affects the sizes and properties of succeeding elements.
25. Similar to lanthanoid contraction, but its effects are less studied because heavier elements are less well known.
26. Lanthanoid contraction is considered more important due to its greater impact on well-studied elements.

One-Line NCERT Facts

• All actinoids are radioactive.
• Actinoids show a wider range of oxidation states (+3 to +7) than lanthanoids.
• Actinoids are more reactive than lanthanoids.
• 5f electrons participate in bonding more readily than 4f electrons.
• Actinoid contraction occurs due to poor shielding by 5f electrons