F-Block Elements | CBSE - Wyatt's Notes

sources:

• text: Standard textbook reference

F-block elements include the lanthanoids (4f series) and actinoids (5f series). They are characterized by the filling of f-orbitals.

• Lanthanoids: La (57) to Lu (71), filling 4f orbitals
• Actinoids: Ac (89) to Lr (103), filling 5f orbitals
• Lanthanoid contraction: steady decrease in ionic radii from La$^{3+}$ to Lu$^{3+}$
• Cause: poor shielding of 4f electrons, so effective nuclear charge increases
• Common oxidation state: +3 for lanthanoids, variable for actinoids (+3 to +7)
• Lanthanoids are paramagnetic (except La$^{3+}$ and Lu$^{3+}$)
• Actinoids are radioactive; only U and Pu have significant quantities in nature
• Colour: due to f-f transitions (partially filled f-orbitals)
• Cerium: Ce$^{4+}$ (oxidizing) and Ce$^{3+}$ (reducing)

Problem: Explain why the ionic radius of Zr$^{4+}$ (79 pm) is very close to Hf$^{4+}$ (78 pm) despite Hf being in the next period.

Solution:

Zr is in Period 5 (Group 4), Hf is in Period 6 (Group 4).

Between them lie the lanthanoids (La to Lu). The lanthanoid contraction causes a steady decrease in atomic/ionic radii across the 4f series.

The cumulative effect of lanthanoid contraction is approximately 20 pm. This nearly cancels the expected increase in radius going from Period 5 to Period 6.

Result: Zr$^{4+}$ and Hf$^{4+}$ have nearly identical sizes, leading to very similar chemistry.

Common mistake: Attributing the similar sizes to periodic trends alone. Without the lanthanoid contraction, Hf$^{4+}$ would be significantly larger than Zr$^{4+}$.

Problem: Write the electron configuration of Gd$^{3+}$ (Z = 64).

Solution:

Gd (Z = 64): $[\text{Xe}] 4f^7 5d^1 6s^2$ (exception to the filling order)

Gd$^{3+}$: remove 3 electrons (2 from 6s, 1 from 5d):

$[\text{Xe}] 4f^7$

This is a half-filled f-subshell, which is particularly stable.

Common mistake: Writing $[\text{Xe}] 4f^8$ instead of $[\text{Xe}] 4f^7$. Remember to remove from the outermost orbitals first (6s, then 5d, then 4f).

Problem: Explain why uranium forms compounds in the +6 oxidation state (e.g., $\text{UF}_6$).

Solution:

Uranium (Z = 92): $[\text{Rn}] 5f^3 6d^1 7s^2$

In +6 state: remove all 6 outer electrons (3 from 5f, 1 from 6d, 2 from 7s), leaving the configuration $[\text{Rn}]$.

The +6 state is possible because:

1. Actinoids have accessible 5f, 6d, and 7s orbitals for bonding
2. The energy difference between 5f and 6d is small (unlike lanthanoids where 4f is much lower)
3. High oxidation states are stabilized by electronegative ligands (F, O)

$\text{UF}_6$ is used in uranium enrichment (gaseous diffusion process) because it sublimes at 56°C.

Common mistake: Assuming actinoids behave like lanthanoids. Actinoids show much more variable oxidation states due to the energy proximity of 5f, 6d, and 7s orbitals.

1. Write the electron configuration of Eu$^{3+}$ (Z = 63).
2. Explain why Ce$^{4+}$ is a stronger oxidizing agent than La$^{3+}$.
3. Why is $\text{UO}_2$ used as a nuclear fuel?

Lanthanoids are used in magnets (Nd$_2$Fe$_{14}$B), electronics (Ce in catalytic converters), and lighting (Eu in phosphors). Actinoids are central to nuclear energy and weapons technology.

The hidden row at the bottom of the periodic table: Lanthanoids and actinoids are like the backstage crew in a theater production — they’re hidden from the main stage (the periodic table’s body) but are essential to the show. The lanthanoid contraction is like a slow squeeze: as you add protons and electrons across the 4f series, the 4f electrons are poor at shielding the nucleus, so each element pulls its electrons in tighter than expected. This squeeze makes Period 6 transition metals almost the same size as their Period 5 counterparts.

Why it matters: Lanthanoids power modern technology — neodymium magnets are in every electric motor and headphone, europium makes phone screens glow red, and cerium cleans car exhaust. Actinoids are central to nuclear energy and medicine (uranium-235 fuels reactors, plutonium-238 powers space probes). Without f-block elements, modern life would look very different.

The key insight: The 4f orbitals are buried deep inside the atom, shielded by 5s and 5p electrons, so they participate little in bonding — which is why lanthanoids almost always show only the +3 oxidation state, unlike d-block metals with their variable oxidation states.

• Lanthanoid contraction affects the chemistry of Period 6 transition metals
• +3 is the most common oxidation state for lanthanoids
• Colour arises from f-f transitions (partially filled f-orbitals)
• Actinoids show more variable oxidation states than lanthanoids
• Electron configurations often have exceptions (Gd, Ce, etc.)

Assuming actinoids behave like lanthanoids. Actinoids show much more variable oxidation states (+3 to +7) because the 5f, 6d, and 7s orbitals are close in energy. Lanthanoids are restricted to mainly +3 because the 4f orbitals are too buried to participate in bonding. This is a fundamental difference, not just a trend variation.

Writing the wrong electron configuration for Gd³⁺. Gd (Z=64) has the exception [Xe]4f⁷5d¹6s². When forming Gd³⁺, you remove 2 electrons from 6s and 1 from 5d, giving [Xe]4f⁷ (half-filled f-subshell), not [Xe]4f⁸. Always remove from outermost orbitals first.

Forgetting that lanthanoid contraction explains the similarity of Zr and Hf. Without the lanthanoid contraction, Hf would be significantly larger than Zr. The cumulative contraction across the 4f series nearly cancels the expected size increase from Period 5 to Period 6. Students often attribute the similarity to general periodic trends alone.

• D-Block Elements: Lanthanoid contraction directly affects Period 6 d-block elements (Zr ≈ Hf, Nb ≈ Ta), connecting f-block trends to d-block chemistry.
• Coordination Compounds: Lanthanoid ions form coordination complexes, though with less variety than d-block metals due to their fixed +3 oxidation state.
• Electrochemistry: The different oxidation states of cerium (Ce³⁺/Ce⁴⁺) are used in redox titrations and electrochemical applications.
• Atoms and Nuclei (Physics): Actinoid radioactive decay and nuclear binding energy connect f-block chemistry to nuclear physics.