Question
Compare the properties of lanthanoids and actinoids. Explain lanthanoid contraction and its consequences. Why do actinoids show a wider range of oxidation states than lanthanoids?
Solution — Step by Step
As we move across the lanthanoid series (La to Lu), electrons are added to 4f orbitals. The 4f electrons are poor shielders — they do not effectively screen the nuclear charge from outer electrons. So the effective nuclear charge increases steadily, causing a gradual decrease in atomic and ionic radii.
This is lanthanoid contraction: ionic radii decrease from La³⁺ (103 pm) to Lu³⁺ (86 pm).
- Post-lanthanoid elements (Hf, Ta, W…) have almost the same radii as their 4d counterparts (Zr, Nb, Mo…) — making their chemistry very similar and separation difficult.
- Similar properties among lanthanoids themselves — all are very alike in chemical behavior, making separation challenging (ion-exchange chromatography is needed).
- Basicity decreases from La(OH)₃ to Lu(OH)₃ (smaller ion = weaker base).
| Property | Lanthanoids (4f) | Actinoids (5f) |
|---|---|---|
| Common oxidation state | +3 (dominant) | +3, but also +4, +5, +6, +7 |
| Shielding by f orbitals | 4f shielding poor but more than 5f | 5f shielding even poorer |
| Radioactivity | Most are non-radioactive | All are radioactive |
| Complex formation | Less tendency | Greater tendency (higher charge density) |
| Contraction | Lanthanoid contraction | Actinoid contraction (similar concept) |
| Examples | La, Ce, Nd, Eu, Gd | Ac, Th, U, Pu, Am |
flowchart TD
A[f-block Elements] --> B[Lanthanoids 4f]
A --> C[Actinoids 5f]
B --> B1[Mostly +3 oxidation state]
B --> B2[Lanthanoid contraction]
B --> B3[Non-radioactive mostly]
C --> C1[Variable oxidation states +3 to +7]
C --> C2[Actinoid contraction]
C --> C3[All radioactive]
B2 --> D[Post-lanthanoid elements similar to 4d]Why This Works
Actinoids show more oxidation states because the energy gap between 5f, 6d, and 7s orbitals is very small — electrons can be removed from any of these. In lanthanoids, the 4f orbitals are more deeply buried, so removing electrons beyond the +3 state requires much more energy. Ce⁴⁺ and Eu²⁺ are notable exceptions among lanthanoids.
Common Mistake
Students say “all lanthanoids show only +3 oxidation state.” Most do, but Ce shows +4 (because Ce⁴⁺ has a noble gas configuration) and Eu shows +2 (because Eu²⁺ has a half-filled 4f⁷ configuration). These exceptions are favourite JEE questions.
The similarity between Zr-Hf and Nb-Ta pairs (due to lanthanoid contraction) is a frequently tested consequence. Zr and Hf have nearly identical ionic radii, making their separation the most difficult in all of chemistry.