f-Block Elements — Lanthanoids and Actinoids

• What lanthanoids and actinoids are, and where they are placed in the periodic table
• The f-orbital filling pattern for both series
• What lanthanoid contraction is, its cause, and its far-reaching effects
• How to compare lanthanoids and actinoids (oxidation states, radioactivity, magnetism)
• Why actinoids show more variable oxidation states than lanthanoids
• Uses of lanthanoids and actinoids in modern technology

What are f-Block Elements?

f-Block elements are those in which the last electron enters an f-orbital:

La (Z=57) and Ac (Z=89) are often included in discussions but have empty f-orbitals.

Lanthanoids — 4f Series

Lanthanoids: La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu

Memory phrase: "La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu"

General electronic configuration: [Xe] 4f^(1–14) 5d^(0–1) 6s²

• Most lanthanoids have 4f electrons filling with 5d⁰ or 5d¹
• Notable exceptions: La ([Xe]5d¹6s²), Ce ([Xe]4f¹5d¹6s²), Gd ([Xe]4f⁷5d¹6s² — half-filled 4f)

Common oxidation state: +3 in all lanthanoids (most stable)

• Ce also shows +4 (CeO₂ — oxidising agent); Eu and Sm show +2

Actinoids — 5f Series

Actinoids: Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr

General electronic configuration: [Rn] 5f^(0–14) 6d^(0–2) 7s²

• All actinoids are radioactive — many are artificially produced transuranium elements
• Th, Pa, U, and Np occur naturally in significant amounts; elements beyond Np (Z=93) are synthetic

Common oxidation states: +3, +4, +5, +6 (actinoids show a wider range than lanthanoids)

• U: +3, +4, +5, +6 (UO₂²⁺ — uranyl ion is the most stable)
• Np, Pu also show +7

Lanthanoid Contraction

Definition: The steady decrease in ionic radii of lanthanoid ions (La³⁺ to Lu³⁺) from 106 pm to 86 pm as the atomic number increases.

Cause:

• As 4f electrons are added across the series (Ce to Lu), nuclear charge increases by +1 each step
• 4f orbitals have poor shielding efficiency — they have a complex radial distribution and shield other electrons (and each other) poorly from the increasing nuclear charge
• The effective nuclear charge experienced by outer electrons increases more than expected → electrons are drawn inward → ionic radius decreases

Magnitude: ~1–2 pm decrease per element across the lanthanoid series (total contraction ~20 pm)

Effects of Lanthanoid Contraction

1. Similar radii of 5d elements (periods 5 and 6): The lanthanoid contraction in the 4f series before the 5d series (period 6) compresses the 5d period 6 atoms so that they have almost identical radii to their period 5 counterparts (4d elements).

   4d element	Radius	5d element	Radius
   Zr (40)	160 pm	Hf (72)	159 pm
   Nb (41)	146 pm	Ta (73)	146 pm
   Mo (42)	139 pm	W (74)	139 pm

   This similarity in size makes pairs like Zr/Hf and Nb/Ta chemically almost identical — very difficult to separate.

2. Basicity decreases across lanthanoid series:

   • Smaller ionic size → greater polarising power of Ln³⁺ → weaker base
   • La(OH)₃ is the strongest base; Lu(OH)₃ is the weakest base

3. Separation of lanthanoids is difficult — their similar sizes give nearly identical properties.

Actinoids vs Lanthanoids — Comparison Table

Why actinoids show more oxidation states:

• 5f, 6d, and 7s orbitals have similar energies in actinoids — all can participate in bonding
• In lanthanoids, the 4f orbitals are deeply buried and not available for bonding; only 5d and 6s contribute
• The energy difference between 5f and 6d is smaller than between 4f and 5d

Uses of Lanthanoids and Actinoids

Lanthanoids:

• Rare-earth magnets (Nd₂Fe₁₄B): Strongest permanent magnets — used in EV motors, headphones, hard drives
• Phosphors: Eu³⁺ (red), Tb³⁺ (green) in LED screens and fluorescent lamps
• Misch metal (Ce, La, Nd alloy): Used in lighter flints, pyrophoric alloys
• CeO₂: Polishing agent for glass; in catalytic converters
• Lanthanoid-doped glass: Used in lasers (Nd:YAG laser for eye surgery)

Actinoids:

• U-235: Nuclear fission fuel in nuclear power plants (fissile material)
• Pu-239: Used in nuclear weapons and as reactor fuel
• Th-232: Fertile material in thorium reactors (India's 3-stage nuclear program)
• Am-241: Used in smoke detectors (α-emitter)

Why 5f Orbitals Are More Available for Bonding Than 4f

In lanthanoids, 4f orbitals are contracted and buried close to the nucleus (deeply core-like):

• They don't participate significantly in chemical bonding
• Crystal field effects on 4f are small
• Magnetic properties are governed by isolated 4f electrons

In actinoids, 5f orbitals are more diffuse and have higher radial extent:

• Energy gap between 5f, 6d, and 7s is small → orbital mixing → more oxidation states
• Actinoid complexes show stronger crystal field effects than lanthanoid complexes
• 5f participation in covalent bonding is possible (especially in UF₆, uranyl ion)

Lanthanoid Contraction — Quantitative JEE Perspective

The total ionic radius decrease from La³⁺ (106.1 pm) to Lu³⁺ (86.1 pm) = 20 pm over 14 elements.

This is due to incomplete shielding — each additional 4f electron adds charge shielding of less than +1 unit (shielding constant for 4f-4f ~ 0.35 per electron, much less than inner-core shielding).

For JEE: The question "Why are Zr and Hf almost identical in size?" has the one-line answer: lanthanoid contraction.

Oxidation State Stability Comparison

Lanthanoids:

• +3 is universally stable (loss of 6s² and one 4f/5d electron)
• Ce(+4): stable because Ce⁴⁺ achieves [Xe] configuration (empty 4f) — extra stability
• Eu(+2): Eu²⁺ has [Xe]4f⁷ — half-filled 4f, extra stable
• Yb(+2): Yb²⁺ has [Xe]4f¹⁴ — completely filled 4f, extra stable

Actinoids:

• Pa(+5), U(+6), Np(+7) — accessible because 5f, 6d, 7s are all close in energy
• Higher OS stabilised by strongly oxidising conditions and electronegative ligands (F, O)
• Stability of +3 increases going from Pa → Lr (similar to lanthanoids towards the end of series)

Colour of f-Block Ions

Both lanthanoid and actinoid ions are often coloured:

• Colour arises from f-f transitions (within 4f or 5f energy levels) and also charge-transfer transitions
• f-f transitions are Laporte-forbidden but weakly observed due to vibronic coupling
• Actinoid colours are more intense than lanthanoid colours due to greater f-orbital involvement

Example 1: Lanthanoid Contraction Explains Zr/Hf Separation Difficulty

Problem: Why are Zr and Hf (period 5 and period 6 of group 4) almost 
         identical in chemical properties and very difficult to separate?

Answer:
Before the 5d series in period 6, the 14 lanthanoid elements 
(Ce to Lu, filling 4f¹ to 4f¹⁴) cause the lanthanoid contraction.

Each lanthanoid adds a 4f electron with poor shielding → increasing
effective nuclear charge → decreasing atomic/ionic radius.

Total contraction ≈ 20 pm across 14 lanthanoids.

This cancels the expected increase in size from period 5 → period 6.

Result:
- Zr (period 5, group 4): atomic radius = 160 pm
- Hf (period 6, group 4): atomic radius = 159 pm  ← nearly identical

Same size → same charge-to-radius ratio → nearly identical chemistry
→ Zr and Hf always occur together in nature and are extremely difficult
  to separate (requires ion exchange chromatography or solvent extraction).

Example 2: Why Actinoids Show More Oxidation States Than Lanthanoids

Problem: Explain why U shows +4, +5, +6 oxidation states while the 
         corresponding lanthanoid Nd shows mainly +3.

Comparison:
   Nd (Z=60): [Xe] 4f⁴ 6s²
   Configuration of 4f is DEEPLY buried in the core
   Energy of 4f >> energy of 5d/6s for bonding purposes
   → Only 5d and 6s electrons available → max OS usually +3

   U (Z=92):  [Rn] 5f³ 6d¹ 7s²
   5f, 6d, and 7s have similar energies in early actinoids
   All these electrons can potentially be involved in bonding
   → U can lose 3, 4, 5, or 6 electrons (OS +3 to +6)
   → UF₆ (U in +6) and UO₂²⁺ (uranyl ion, U in +6) are stable

Conclusion: 5f orbital energy similarity to 6d/7s in actinoids 
enables wider range of oxidation states.

1. How many elements are in the lanthanoid series? Name the first and last.
2. What is the cause of lanthanoid contraction?
3. Give one consequence of lanthanoid contraction on 5d transition metals.
4. Why are all actinoids radioactive but most lanthanoids are not?
5. (Stretch) Ce shows a +4 oxidation state and Eu shows a +2 oxidation state. Using the concept of extra stability of empty, half-filled, and completely filled f-subshells, explain why each shows this extra oxidation state.

• NCERT Class 12, Chapter 8, Sections 8.5 (Lanthanoids) and 8.6 (Actinoids)
• Table 8.6 and 8.7 (NCERT) — lanthanoid and actinoid properties
• JEE Main: Direct MCQs on lanthanoid contraction definition, effects, actinoid vs lanthanoid OS comparison
• JEE Advanced: Explanation-type questions on why 5d properties resemble 4d, why actinoids show variable OS
• NEET: Definition of lanthanoid contraction, uses (nuclear reactors, magnets, phosphors)

Study strategy: Draw a flowchart: Lanthanoid contraction (cause: poor 4f shielding) → effects (smaller Ln³⁺ radii, similar 4d/5d radii, harder to separate pairs like Zr/Hf, decrease in basicity). The comparison table between lanthanoids and actinoids is high-yield — memorise it as a table.

Work through the f-Block Elements question sets in Chemistry D-Block chapter. Start with Easy (definitions, uses), then Medium (lanthanoid contraction effects, OS comparison), then Hard (JEE-level electronic configuration and OS reasoning).

Ask: "Can you show me a step-by-step explanation of why Zr and Hf have nearly the same atomic radius, using lanthanoid contraction?" for a visual diagram with numbers.

Complete all 5 Quick Check questions. If Q5 is unclear, revisit the "Oxidation State Stability" section under Level 2. Track your f-block mastery score in Drishti before moving to revision.

• Revise: Full D-Block chapter summary (transition metals + properties + f-block)
• Practice: Mixed JEE-level MCQs across all three d-block notes (Hard difficulty)
• Next chapter: Coordination Compounds (ligands, IUPAC naming, isomerism, crystal field theory)