Actinoids: The 5f Series

Introduction

Actinoids (also called actinides) are the 14 radioactive elements in the 5f series - from Thorium (Th) to Lawrencium (Lr). These are the heaviest naturally occurring elements, and most are synthetic! They’re crucial for nuclear energy and weapons.

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Position in Periodic Table

Actinoid Series:

• Elements: Th (90) to Lr (103) - 14 elements
• Also includes: Actinium (Ac, 89) - often grouped with them
• Period: 7th period
• Block: f-block (5f orbitals being filled)

graph TD
    A[f-Block: Inner Transition] --> B[Lanthanoids: 4f]
    A --> C[Actinoids: 5f]
    B --> B1[Ce to Lu: 58-71]
    B --> B2[Natural, Non-radioactive]
    C --> C1[Th to Lr: 90-103]
    C --> C2[ALL Radioactive]

    style C2 fill:#e74c3c

In Periodic Table:

• Placed below lanthanoids
• Interrupt the 6d series
• Ac → 5f fills → Lr → 6d continues

Interactive Demo: Visualize Actinoid Elements in Periodic Table

Explore the position of actinoids and their electronic configurations.

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Electronic Configuration

General Configuration

$$\boxed{[Rn] \, 5f^{0-14} \, 6d^{0-2} \, 7s^2}$$

Complete Electronic Configurations:

Element	Z	Symbol	Electronic Configuration	f-electrons	Half-life
Actinium	89	Ac	[Rn]6d¹7s²	0	21.8 years
Thorium	90	Th	[Rn]6d²7s²	0	1.4×10¹⁰ y
Protactinium	91	Pa	[Rn]5f²6d¹7s²	2	3.3×10⁴ y
Uranium	92	U	[Rn]5f³6d¹7s²	3	4.5×10⁹ y
Neptunium	93	Np	[Rn]5f⁴6d¹7s²	4	2.1×10⁶ y
Plutonium	94	Pu	[Rn]5f⁶7s²	6	2.4×10⁴ y
Americium	95	Am	[Rn]5f⁷7s²	7	7370 y
Curium	96	Cm	[Rn]5f⁷6d¹7s²	7	1.6×10⁷ y
Berkelium	97	Bk	[Rn]5f⁹7s²	9	1380 y
Californium	98	Cf	[Rn]5f¹⁰7s²	10	898 y
Einsteinium	99	Es	[Rn]5f¹¹7s²	11	472 days
Fermium	100	Fm	[Rn]5f¹²7s²	12	100 days
Mendelevium	101	Md	[Rn]5f¹³7s²	13	52 days
Nobelium	102	No	[Rn]5f¹⁴7s²	14	58 min
Lawrencium	103	Lr	[Rn]5f¹⁴6d¹7s²	14	27 sec

Irregularities:

The filling of 5f and 6d orbitals is VERY irregular because:

1. 5f, 6d, and 7s orbitals have very similar energies
2. Relativistic effects become significant
3. Nuclear instability affects electronic structure

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General Characteristics

1. Radioactivity

ALL actinoid elements are radioactive - the defining characteristic!

Types of Decay:

• α-decay: Emission of He²⁺ nucleus (most common)
• β-decay: Electron emission
• Spontaneous fission: Heavy nuclei split

Examples:

$$^{238}U \rightarrow ^{234}Th + ^4He \quad \text{(α-decay)}$$ $$^{239}Pu \rightarrow ^{235}U + ^4He \quad \text{(α-decay)}$$

Natural vs Synthetic:

Natural	Synthetic
Th-232	Np-237 onwards
Pa-231	Transuranic elements
U-235, U-238	Made in nuclear reactors

Half-lives:

• Th-232: 1.4 × 10¹⁰ years (longer than Earth’s age!)
• U-238: 4.5 × 10⁹ years
• Pu-239: 24,000 years
• Nobelium: 58 minutes
• Lawrencium: 27 seconds

2. Oxidation States

Actinoids show MUCH MORE variable oxidation states than lanthanoids!

Range: +3 to +7 (much broader than lanthanoids)

Oxidation State Chart:

Element	Common OS	Range	Most Stable
Ac	+3	+3	+3
Th	+4	+3, +4	+4
Pa	+5	+3, +4, +5	+5
U	+4, +6	+3, +4, +5, +6	+6 (in UO₂²⁺)
Np	+5	+3, +4, +5, +6, +7	+5
Pu	+4	+3, +4, +5, +6, +7	+4
Am	+3	+3, +4, +5, +6	+3
Cm	+3	+3, +4	+3
Later	+3	+3	+3

Trends:

1. Early actinoids (Th-Pu): Show +4, +5, +6, +7 states
2. Later actinoids (Am onwards): Predominantly +3 (like lanthanoids)
3. +3 state becomes more stable across the series

Why more variable than lanthanoids?

1. 5f, 6d, 7s orbitals have similar energies
2. All three can participate in bonding
3. Larger atomic size allows multiple states
4. Less shielding → electrons more available

3. Actinoid Contraction

Similar to lanthanoid contraction, but more irregular.

$$\boxed{\text{Ionic size: } Th^{4+} > Pa^{4+} > U^{4+} > ... > Lr^{3+}}$$

Cause:

• Poor shielding by 5f electrons
• Increasing effective nuclear charge
• Pulls outer electrons closer

Difference from Lanthanoid Contraction:

• More irregular due to variable oxidation states
• Less pronounced per element
• Complicated by nuclear instability

4. Magnetic Properties

Most actinoids are paramagnetic due to unpaired 5f electrons.

Examples:

• U³⁺ (5f³): 3 unpaired e⁻, paramagnetic
• U⁴⁺ (5f²): 2 unpaired e⁻, paramagnetic
• Pu⁴⁺ (5f⁴): 4 unpaired e⁻, paramagnetic

Complexity:

• Magnetic moments don’t follow simple formulas
• Orbital contributions are significant
• Spin-orbit coupling is strong

5. Color

Actinoid ions are deeply colored (more intense than lanthanoids).

Colors in Aqueous Solution:

Ion	Color	Ion	Color
U³⁺	Red	Pu³⁺	Blue-violet
U⁴⁺	Green	Pu⁴⁺	Yellow-brown
UO₂²⁺	Yellow	PuO₂²⁺	Pink
Np³⁺	Purple	Am³⁺	Pink
Np⁴⁺	Yellow-green	Cm³⁺	Colorless

Why deeper colors than lanthanoids?

• 5f-5f transitions
• 5f-6d transitions (possible due to energy similarity)
• Charge-transfer transitions
• More intense absorption

6. Complex Formation

Actinoids form complexes MORE readily than lanthanoids.

Reasons:

1. Higher oxidation states (+4, +5, +6)
2. Larger charge → stronger electrostatic attraction
3. 5f, 6d orbitals can participate in bonding
4. Greater covalent character

Important Complexes:

Complex	Name	Use
UO₂(NO₃)₂	Uranyl nitrate	Fuel processing
[UF₆]	Uranium hexafluoride	U-235 enrichment
[PuF₆]	Plutonium hexafluoride	Pu separation
ThF₄	Thorium fluoride	Nuclear reactors

Coordination Numbers:

• Usually high: 6, 8, 10, 12
• Due to large ionic size
• Depends on oxidation state

7. Chemical Reactivity

Actinoids are quite reactive (similar to lanthanoids).

Reactions:

With air/oxygen:

$$4Ac + 3O_2 \rightarrow 2Ac_2O_3$$

With water:

$$2Ac + 6H_2O \rightarrow 2Ac(OH)_3 + 3H_2$$

With acids:

$$2Ac + 6HCl \rightarrow 2AcCl_3 + 3H_2$$

With halogens:

$$Th + 2F_2 \rightarrow ThF_4$$

Reactivity decreases across the series (like lanthanoids).

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Important Actinoid Elements

1. Thorium (Th, Z=90)

Properties:

• Most abundant actinoid
• Silvery-white metal
• Oxidation state: +4
• Less radioactive (very long half-life)

Occurrence:

• Monazite sand (ThO₂)
• More abundant than uranium

Uses:

• Nuclear fuel (Th-232 → U-233 breeding)
• Gas mantles (thoriated tungsten)
• Alloys (Mg-Th alloys)

Compounds:

• ThO₂: Refractory material (m.p. 3300°C)
• ThF₄: Nuclear applications

2. Uranium (U, Z=92)

Properties:

• Silvery-white, dense, radioactive metal
• Common oxidation states: +4, +6
• Three natural isotopes: U-234, U-235, U-238

Isotopes:

• U-238 (99.3%): Not fissile, fertile (→ Pu-239)
• U-235 (0.7%): Fissile, used in reactors and weapons
• U-234 (trace): Decay product

Uranium Enrichment:

Natural uranium must be enriched to increase U-235 content.

Method: Gaseous diffusion of UF₆

• Lighter ²³⁵UF₆ diffuses slightly faster than ²³⁸UF₆
• Repeated many times to enrich U-235

Compounds:

Uranyl ion (UO₂²⁺):

• Most stable form in solution
• Linear structure: O=U=O
• Yellow color

UF₆ (Uranium hexafluoride):

• Colorless solid
• Sublimes at 56°C
• Used for enrichment

U₃O₈:

• “Yellowcake” uranium ore
• Starting material for processing

Uses:

• Nuclear fuel (reactors and weapons)
• Dating (U-238 → Pb-206 dating)
• Depleted uranium (armor-piercing shells)

3. Plutonium (Pu, Z=94)

Properties:

• Silvery metal (tarnishes to yellow)
• Highly radioactive and toxic
• Common oxidation states: +3, +4, +5, +6
• Synthetic (not naturally occurring)

Production:

Pu-239 produced in nuclear reactors:

$$^{238}U + n \rightarrow ^{239}U \xrightarrow{\beta^-} ^{239}Np \xrightarrow{\beta^-} ^{239}Pu$$

Isotopes:

• Pu-239: Fissile, used in weapons and reactors
• Half-life: 24,100 years

Uses:

• Nuclear weapons (critical mass ~10 kg)
• Nuclear reactors (MOX fuel)
• RTGs (Radioisotope thermoelectric generators in spacecraft)
• Pacemakers (older models)

Danger:

• Extremely toxic (α-emitter)
• Long half-life (environmental hazard)
• Can reach critical mass

4. Americium (Am, Z=95)

Properties:

• Silvery-white metal
• Oxidation state: +3 (most stable)
• Synthetic element

Isotope:

• Am-241: Half-life 432 years
• α-emitter

Uses:

• Smoke detectors! (Am-241 source)
• Industrial gauges
• Medical diagnosis

Everyday Application:

Your smoke detector contains ~0.3 μg of Am-241!

• α-particles ionize air
• Smoke interrupts current → alarm sounds

5. Curium (Cm, Z=96)

Named after: Marie and Pierre Curie

Properties:

• Synthetic
• Glows in the dark (self-heating from radioactivity)
• Oxidation state: +3

Isotope:

• Cm-244: Used in RTGs

Uses:

• Alpha-particle X-ray spectrometers (Mars rovers!)
• RTGs for space missions

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Lanthanoids vs Actinoids: Detailed Comparison

Property	Lanthanoids (4f)	Actinoids (5f)
General Formula	[Xe]4f¹⁻¹⁴5d⁰⁻¹6s²	[Rn]5f⁰⁻¹⁴6d⁰⁻²7s²
Radioactivity	Only Pm (all isotopes)	ALL elements
Oxidation States	Predominantly +3	+3 to +7 (variable)
Common OS	+3	+3, +4, +5, +6
Color Intensity	Pale colors	Deep, intense colors
Magnetic Moment	Calculated (complex)	More complex
Complex Formation	Less tendency	Greater tendency
Ionic Radii (M³⁺)	Larger	Smaller
Contraction	Regular	More irregular
Occurrence	Natural (except Pm)	Th, Pa, U natural; rest synthetic
f-orbital Bonding	Minimal	Significant (5f, 6d similar energy)
Industrial Use	Magnets, LEDs, catalysts	Nuclear energy, weapons

Why Actinoids More Variable?

graph TD
    A[Actinoids More Variable] --> B[5f, 6d, 7s close in energy]
    A --> C[Larger atomic size]
    A --> D[Relativistic effects]
    B --> E[All orbitals participate in bonding]
    C --> F[Can accommodate multiple oxidation states]
    D --> G[Orbital energies affected]

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Applications and Nuclear Chemistry

Nuclear Fission

Fission of U-235:

$$^{235}U + n \rightarrow ^{236}U^* \rightarrow \text{Fission products} + 2-3 \, n + \text{Energy}$$

Example:

$$^{235}U + n \rightarrow ^{141}Ba + ^{92}Kr + 3n + 200 \text{ MeV}$$

Chain Reaction:

• Each fission releases 2-3 neutrons
• These cause more fissions
• Exponential growth → explosion OR controlled power

Nuclear Reactors

Fuel: U-235 (enriched) or Pu-239

Components:

1. Fuel rods: UO₂ pellets
2. Moderator: Slows neutrons (H₂O, D₂O, graphite)
3. Control rods: Absorb neutrons (Cd, B)
4. Coolant: Removes heat

Breeder Reactors:

Convert fertile material to fissile:

$$^{238}U + n \rightarrow ^{239}Pu \quad \text{(fertile → fissile)}$$ $$^{232}Th + n \rightarrow ^{233}U \quad \text{(fertile → fissile)}$$

Nuclear Weapons

Fission Bombs:

• Use U-235 or Pu-239
• Critical mass brought together rapidly
• Uncontrolled chain reaction

Critical Mass:

• U-235: ~50 kg
• Pu-239: ~10 kg (lower, more dangerous)

Fusion Bombs (Thermonuclear):

• Fission bomb triggers fusion
• Fusion of deuterium/tritium
• Much more powerful

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Common JEE Mistakes

1. Assuming actinoids = lanthanoids

   • They’re different! Actinoids show MUCH more variable oxidation states
   • ALL actinoids are radioactive (only Pm in lanthanoids)

2. Forgetting transuranic elements

   • Elements after U (Z=92) are ALL synthetic
   • Made in nuclear reactors or particle accelerators

3. U-235 vs U-238 confusion

   • U-235: Fissile (0.7%)
   • U-238: Fertile, more abundant (99.3%)

4. Oxidation state trends

   • Early actinoids (Th-Pu): Variable (+3 to +7)
   • Later actinoids (Am onwards): Predominantly +3

5. Americium application

   • It’s in smoke detectors! (Am-241)
   • Common MCQ answer

6. Complex formation

   • Actinoids form complexes MORE readily than lanthanoids
   • Due to higher oxidation states and 5f orbital participation

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Practice Problems

Level 1: Basic Concepts

1. What is the key difference between lanthanoids and actinoids?

2. Why are all actinoids radioactive?

3. Write electronic configuration of:

   • Th (Z=90)
   • U (Z=92)
   • Pu (Z=94)

4. Name three naturally occurring actinoids.

Level 2: Application

5. Explain why:

   • Actinoids show more oxidation states than lanthanoids
   • U-235 is used in nuclear reactors, not U-238
   • Pu-239 is more dangerous than U-235 for weapons

6. Compare:

   • Complex formation tendency of lanthanoids and actinoids
   • Color intensity of Ln³⁺ and An³⁺ ions
   • Oxidation state variability

7. How is Pu-239 produced from U-238?

Level 3: JEE Advanced

8. The most common oxidation state shown by actinoid elements is:

   • (a) +2
   • (b) +3
   • (c) +4
   • (d) +6

9. Which statement is INCORRECT?

   • (a) All actinoids are radioactive
   • (b) Actinoids show oxidation states from +3 to +7
   • (c) Actinoid contraction is similar to lanthanoid contraction
   • (d) All actinoids are naturally occurring

10. Arrange in order of increasing stability of +3 oxidation state: Th, U, Pu, Am

11. Assertion (A): Actinoids form complexes more readily than lanthanoids. Reason (R): 5f orbitals participate in bonding in actinoids.

    • (a) Both A and R true, R explains A
    • (b) Both true, R doesn’t explain A
    • (c) A true, R false
    • (d) Both false

12. Uranyl ion (UO₂²⁺) is:

    • (a) Linear
    • (b) Bent
    • (c) Tetrahedral
    • (d) Octahedral

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Memory Tricks

“TRAP” for Actinoid Properties

Actinoids are:

• Transuranic (most are synthetic)
• Radioactive (ALL of them)
• All show variable oxidation states (+3 to +7)
• Paramagnetic (mostly)

Important Elements

“ThUPAC” for key actinoids:

• Thorium - +4, nuclear fuel
• Uranium - +6 (UO₂²⁺), fission
• Plutonium - weapons, MOX fuel
• Americium - smoke detectors
• Curium - RTGs, named after Curies

Oxidation States

“Early Variable, Later Three”

• Th to Pu: Variable (+3 to +7)
• Am onwards: Predominantly +3

vs Lanthanoids

“ARCO” - Actinoids vs Lanthanoids:

• ALL radioactive (lanthanoids: only Pm)
• Rich in oxidation states (lanthanoids: mainly +3)
• Complexes more readily (lanthanoids: less)
• Orbitals (5f) participate in bonding (lanthanoids: 4f don’t)

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Related Topics

Within d-f Block Elements

• Transition Elements - d-block comparison
• Properties of d-Block - Oxidation states
• Lanthanoids - Direct comparison with 4f series
• Important Compounds - Oxidizing agents

Other Chemistry Topics

• Atomic Structure - Electronic configuration
• Radioactivity - Decay processes
• Chemical Bonding - Complex formation

Cross-Subject Connections

• Nuclear Physics - Fission, fusion
• Modern Physics - Radioactive decay

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