Transition &
Inner Transition Elements
A deep-dive into the remarkable chemistry of d-block transition metals and f-block lanthanides & actinides — their electronic structures, characteristic properties, and JEE/NEET exam essentials.
What are Transition Elements?
Definition
Transition elements (d-block) are elements in which the last electron enters a d-subshell of the penultimate (inner) energy level. They are found in Groups 3–12 of the periodic table, spanning Periods 4 through 7.
IUPAC defines a transition element as one which has an incomplete d-subshell either in its ground state or in any of its commonly occurring oxidation states. This excludes Zn, Cd, and Hg (d¹⁰ in all oxidation states).
d-Block vs Transition Metal
All transition metals are d-block elements, but not all d-block elements are transition metals. Zn (d¹⁰s²), Cd, and Hg have completely filled d-orbitals in their ground state and all stable oxidation states, so they are d-block but NOT transition metals.
Series of Transition Elements
3d series: Sc (21) → Zn (30) [Period 4]
4d series: Y (39) → Cd (48) [Period 5]
5d series: La (57), Hf (72) → Hg (80) [Period 6]
6d series: Ac (89), Rf (104)→ onwards [Period 7]
Electronic Configuration of d-Block
General Configuration
The general electronic configuration of transition elements is:
Where n is the outermost (valence) shell. The (n-1)d orbitals are filled with 1 to 10 electrons while the outermost ns orbital contains 1 or 2 electrons.
- Cr: [Ar] 3d⁵ 4s¹ (NOT 3d⁴ 4s²) — half-filled d stability
- Cu: [Ar] 3d¹⁰ 4s¹ (NOT 3d⁹ 4s²) — fully-filled d stability
- During ionization, 4s electrons are removed BEFORE 3d electrons
- Fe²⁺: [Ar] 3d⁶ | Fe³⁺: [Ar] 3d⁵ (more stable — half-filled)
- Cu²⁺ is more stable than Cu⁺ due to greater hydration enthalpy
Variable Oxidation States
Why do transition elements show variable oxidation states?
Transition metals exhibit multiple oxidation states because both (n-1)d and ns electrons can participate in bond formation. The energy difference between (n-1)d and ns subshells is small, making it easy to involve different numbers of electrons in bonding.
The oxidation states range from +2 (minimum for most) to the group number (maximum). As we go from left to right in the series, maximum oxidation state first increases then decreases.
Key Points on Oxidation States
• Maximum oxidation state = group number (up to Mn, +7)
• After Mn, maximum ox. state decreases
• +2 is common minimum (loss of 2 ns electrons)
• Mn shows maximum range: +2 to +7
• Cu shows +1 and +2; Cu⁺ is stable in solid, Cu²⁺ in solution
Stability of Oxidation States
• Stability depends on ionic potential, hydration energy, and lattice energy
• Fe³⁺ (d⁵) is more stable than Fe²⁺ due to half-filled configuration
• Mn²⁺ (d⁵) is more stable than Mn³⁺
• Higher oxidation states are stabilized by oxide/fluoride (electronegative) ligands
- Mn has the maximum number of oxidation states (+2 to +7) in 3d series
- Sc and Zn show only one oxidation state (+3 and +2 respectively)
- During ionization: 4s removed first, then 3d
- Higher oxidation states appear in oxides and fluorides; lower in sulphides and iodides
- Cr(VI) and Mn(VII) are powerful oxidizing agents
Coloured Ions
Cause of Colour — d-d Transition
Transition metal ions are coloured because they can absorb visible light, causing electrons to jump from a lower d orbital to a higher d orbital. The complementary colour of the absorbed light is observed.
Condition for colour: The metal ion must have an incomplete d-subshell (partially filled d orbitals). Ions with d⁰ or d¹⁰ configurations are colourless because no d-d transitions are possible.
Colourless Ions (d⁰ / d¹⁰)
No d-d transition possible:
• Sc³⁺ (d⁰) — colourless
• Ti⁴⁺ (d⁰) — colourless
• Cu⁺ (d¹⁰) — colourless
• Zn²⁺ (d¹⁰) — colourless
Colour & Crystal Field
The exact colour depends on:
• Nature of the metal ion
• Oxidation state
• Nature of ligand
• Geometry of complex
(Crystal Field Theory)
Charge Transfer Colour
Some intense colours are NOT due to d-d transitions but charge transfer (CT):
• MnO₄⁻ (d⁰) — violet
• CrO₄²⁻ (d⁰) — yellow
• Cr₂O₇²⁻ (d⁰) — orange
Paramagnetic Properties
Origin of Magnetic Behaviour
Magnetic properties arise from the spin and orbital motion of electrons. Transition metals and their compounds are often paramagnetic because they contain unpaired electrons in d orbitals.
Each unpaired electron acts as a tiny magnet. In a magnetic field, unpaired electrons align with the field, causing the substance to be attracted — this is paramagnetism.
Types of Magnetic Behaviour
Diamagnetic: All electrons paired → repelled from magnetic field (e.g. Zn²⁺, Cu⁺)
Paramagnetic: Unpaired electrons → weakly attracted to magnetic field. Lost when field is removed.
Ferromagnetic: Special case (Fe, Co, Ni) — very strong attraction, permanent magnetism due to domain alignment.
Antiferromagnetic: Adjacent unpaired electrons aligned anti-parallel → no net magnetism (e.g. MnO).
Catalytic Properties
Why are Transition Metals Good Catalysts?
Transition metals and their compounds are excellent catalysts due to two main reasons: (1) Variable oxidation states — they can gain/lose electrons easily, enabling oxidation-reduction cycles in reactions. (2) Ability to adsorb reactants — their large surface area and unfilled d-orbitals allow reactants to be adsorbed on the metal surface, bringing them closer and lowering activation energy.
Haber Process
Fe catalyst with Al₂O₃, K₂O promoters. Used in synthesis of NH₃ from N₂ and H₂. Fe adsorbs and weakens N≡N bond.
Ostwald Process
Pt-Rh catalyst in the oxidation of NH₃ to NO during manufacture of nitric acid. Pt is a superb heterogeneous catalyst.
Contact Process
V₂O₅ catalyst in oxidation of SO₂ to SO₃ for sulphuric acid manufacture. V⁵⁺ → V⁴⁺ → V⁵⁺ cycle.
Hydrogenation
Ni, Pt, Pd catalysts used in hydrogenation of unsaturated fats and oils. Ni is used in vanaspati production.
Biological Catalysis
Fe in haemoglobin, haem enzymes. Mn and Zn in metalloenzymes. Co in vitamin B₁₂. Mo in nitrogenase.
Ziegler-Natta
TiCl₄ with Al(C₂H₅)₃ for polymerisation of ethylene and propylene to produce polyethene and polypropene.
Formation of Alloys
Why do Transition Metals Form Alloys?
Transition metals readily form alloys because they have similar atomic radii and crystal structures. This allows atoms of one metal to substitute atoms of another in the metallic lattice without disturbing the crystal structure significantly.
Alloys are harder, stronger, and have higher melting points than pure metals. They also show greater resistance to corrosion.
| Alloy | Composition | Key Properties | Application |
|---|---|---|---|
| Stainless Steel | Fe + Cr (12-20%) + Ni | Corrosion resistant, hard, lustrous | Cutlery, surgical instruments, construction |
| Chrome Steel | Fe + Cr (4-18%) | Very hard, cutting edge quality | Cutting tools, ball bearings |
| Nichrome | Ni + Cr + Fe | High electrical resistance, oxidation resistant | Heating elements, resistance wires |
| Alnico | Al + Ni + Co + Fe | Strong permanent magnet | Magnets in motors and speakers |
| Tungsten Steel | Fe + W | Extremely hard, retains hardness at high T | High-speed cutting tools |
| Bronze | Cu + Sn (4-8%) | Hard, corrosion resistant | Coins, statues, ship propellers |
| Brass | Cu + Zn (20-40%) | Malleable, corrosion resistant | Musical instruments, plumbing |
- Alloys form because transition metals have similar sizes — atomic radii range ≈ 124–135 pm
- Alloys are homogeneous solid mixtures — NOT compounds (no fixed stoichiometry)
- Stainless steel: Fe + Cr + Ni (most important alloy for exams)
- Addition of Cr to steel prevents corrosion by forming a protective oxide layer (Cr₂O₃)
- Alnico = Aluminium + Nickel + Cobalt → used as permanent magnets
Formation of Interstitial Compounds
What are Interstitial Compounds?
Interstitial compounds are formed when small non-metallic atoms (H, B, C, N) occupy the interstitial (void) spaces in the lattice of transition metals. These are non-stoichiometric compounds and retain metallic lustre and conductivity.
Properties of Interstitial Compounds
• Very hard (some are harder than the metal itself)
• High melting points (higher than parent metal)
• Retain metallic conductivity and lustre
• Chemically inert (resistant to acids)
• Non-stoichiometric composition
Examples of Interstitial Compounds
Carbides: Fe₃C (cementite, steel hardening), TiC, WC (cutting tools)
Nitrides: TiN, VN (hard coatings)
Hydrides: TiH₁.₇₃, PdH₀.₉₉ (H₂ storage)
Borides: TiB₂, CrB (extreme hardness)
Formation of Complexes
Why do Transition Metals Form Complexes?
Transition metals have a strong tendency to form coordination complexes because they have: (1) small ionic size with a (2) high charge density, and (3) vacant d-orbitals that can accept lone pairs from ligands (Lewis bases).
Important Complexes (JEE Favourites)
• [Fe(CN)₆]⁴⁻ : Ferrocyanide (yellow) — Fe(II), CN⁻ ligands
• [Fe(CN)₆]³⁻ : Ferricyanide (red) — Fe(III)
• [Cu(NH₃)₄]²⁺ : Tetraamminecopper(II) — deep blue
• [Ni(CO)₄] : Tetracarbonylnickel(0) — tetrahedral, used in Mond's process
• [PtCl₄]²⁻ : Tetrachloroplatinate(II) — square planar
Key Features of Complex Formation
• Central metal = Lewis acid; Ligand = Lewis base
• Coordination number = number of ligands bonded
• Effective Atomic Number (EAN) rule: total electrons = noble gas configuration
• Crystal Field Theory explains colour and magnetism of complexes
• Strong field ligands → low spin; weak field → high spin
- CO is the strongest field ligand; F⁻ and I⁻ are the weakest
- Spectrochemical series: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ < CO
- Square planar geometry common with d⁸ ions (Ni²⁺, Pd²⁺, Pt²⁺, Au³⁺)
- Mond's process: Ni + 4CO → [Ni(CO)₄] at 60°C → decomposes at 180°C to pure Ni
- EDTA is a hexadentate ligand (chelating, forms very stable complexes)
Inner Transition Elements
The f-block elements — lanthanides (4f) and actinides (5f) — where the last electron enters an f-orbital. Also called inner transition elements as f-orbitals are internal to the valence shell.
Lanthanides (4f Elements)
Overview of Lanthanides
Lanthanides are the 14 elements from Cerium (Ce, Z=58) to Lutetium (Lu, Z=71), following Lanthanum (Z=57). They are also called Rare Earth Elements, though they are actually not that rare in the Earth's crust.
General Electronic Configuration
General formula: [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s²
The 4f orbitals are progressively filled from Ce to Lu.
Exceptions: Gd ([Xe] 4f⁷ 5d¹ 6s²) — half-filled f stability
Eu ([Xe] 4f⁷ 6s²) and Yb ([Xe] 4f¹⁴ 6s²) have no 5d electrons.
Physical Properties
• Silvery white, lustrous, soft metals
• High reactivity — tarnish in air
• React with H₂O (slowly at RT, rapidly with hot H₂O)
• React with acids to liberate H₂
• Paramagnetic due to unpaired f electrons
• Density: 6.1 (La) to 9.8 (Lu) g/cm³
Oxidation States of Lanthanides
+3 — The Characteristic Oxidation State
The most common and stable oxidation state of lanthanides is +3, formed by loss of two 6s electrons and one 4f (or 5d) electron. This stability is due to the very stable [Xe] core after losing 3 electrons.
However, some lanthanides also show +2 and +4 oxidation states when doing so gives them extra stability (empty, half-filled, or fully filled f-subshell).
+4 Oxidation State
Shown by elements where losing 4e⁻ gives f⁰ or f⁷:
Ce⁴⁺ (f⁰) — very stable, strong oxidant
Tb⁴⁺ (f⁷) — half-filled, less stable
Pr⁴⁺, Nd⁴⁺ — rare, strong oxidants
+3 Oxidation State
All lanthanides form M³⁺ ions. This is the most stable state for virtually all lanthanides. The Ln³⁺ ions have characteristic colours arising from f-f transitions (Laporte forbidden but weakly allowed).
+2 Oxidation State
Shown by elements where losing 2e⁻ gives f⁷ or f¹⁴:
Eu²⁺ (f⁷) — most stable, half-filled
Yb²⁺ (f¹⁴) — fully filled, fairly stable
Sm²⁺ — known but strong reducing agent
Chemical Properties of Lanthanides
- Reaction with O₂: 4Ln + 3O₂ → 2Ln₂O₃ (heated) — oxides are basic in nature
- Reaction with H₂O: 2Ln + 6H₂O → 2Ln(OH)₃ + 3H₂ (readily, except Ce which forms CeO₂)
- Reaction with Halogens: 2Ln + 3X₂ → 2LnX₃ (trihalides)
- Reaction with H₂: Ln + H₂ → LnH₂ or LnH₃ (non-stoichiometric hydrides)
- Reduction power: La³⁺/La = −2.52 V → act as strong reducing agents
- Complex formation: Limited, much less than transition metals; prefer O-donor ligands (EDTA)
Lanthanide Contraction
Definition
Lanthanide contraction refers to the steady decrease in atomic and ionic radii of lanthanide elements from La to Lu as the atomic number increases. Despite the addition of 14 electrons (one by one) from La to Lu, the atomic/ionic size does not increase — instead it decreases continuously.
Cause of Lanthanide Contraction
As we move from La (Z=57) to Lu (Z=71), each element adds one 4f electron AND one proton. The 4f orbitals have very poor shielding ability — they are diffuse and do not effectively shield outer electrons from the increasing nuclear charge. Thus, as nuclear charge increases from 57 to 71 (ΔZ = +14), the poor shielding of 4f electrons means the effective nuclear charge felt by the outermost 6s electrons increases significantly, pulling them closer to the nucleus. This causes a steady decrease in ionic radius (~20 pm total from La³⁺ to Lu³⁺).
Effects of Lanthanide Contraction
1. Similarity of 4d and 5d Transition Metals
The lanthanide contraction makes the atomic radii of 5d-series metals almost equal to those of their 4d counterparts. For example:
Zr (160 pm) ≈ Hf (159 pm)
Nb (146 pm) ≈ Ta (146 pm)
Mo (139 pm) ≈ W (139 pm)
These pairs are extremely difficult to separate chemically!
2. Separation of Lanthanides
Because all lanthanides are very similar in size and chemical properties (all show +3 state), their separation is extremely difficult. Techniques like Ion exchange chromatography and solvent extraction are needed. La and Lu differ by only ~17 pm in M³⁺ radius.
3. Basicity of Lanthanide Hydroxides
As the ionic size decreases from La to Lu, the ionic potential (charge/radius) increases. This means the hydroxides become progressively less basic and more soluble:
La(OH)₃ (most basic) → Lu(OH)₃ (least basic)
La(OH)₃ is basic; some later ones are nearly amphoteric.
4. Increase in Density and Hardness
As atomic size decreases across the series, density increases from La (6.1 g/cm³) to Lu (9.8 g/cm³). Melting points and hardness also generally increase, though with some irregularities due to f-electron bonding effects.
5. Stability of Higher Oxidation States
As ionic size decreases across the series, higher oxidation states become harder to achieve. Ce⁴⁺ is stable, but only Pr and Tb among the later lanthanides can sometimes achieve +4. The lanthanide contraction makes the +3 state increasingly stabilized.
6. Difference in Properties of Ln & An
The lanthanide contraction contributes to the greater separation between 4f and 5f elements compared to within each series. The actinide contraction is slightly less regular due to relativistic effects and broader energy range of 5f orbitals.
- Cause: Poor shielding of 4f electrons → high effective nuclear charge on outer electrons
- Effect on 5d metals: Zr ≈ Hf, Nb ≈ Ta, Mo ≈ W in atomic radius (hardest element pairs to separate)
- Basicity decreases from La(OH)₃ to Lu(OH)₃ (inverse of ionic size)
- Total contraction across lanthanide series ≈ 17–20 pm for M³⁺ ions
- Lanthanide contraction causes 5d elements to be similar in size to their 4d counterparts — key reason for pair similarities
Actinides (5f Elements)
Overview of Actinides
Actinides are the 14 elements from Thorium (Th, Z=90) to Lawrencium (Lr, Z=103), following Actinium (Z=89). All actinides are radioactive. Elements beyond Uranium (Z=92) are synthetic (transuranium elements) produced artificially in nuclear reactions.
General Electronic Configuration of Actinides
General formula: [Rn] 5f¹⁻¹⁴ 6d⁰⁻² 7s²
Unlike lanthanides (where 5d involvement is minimal), actinides involve both 5f and 6d orbitals extensively in bonding, because the energy difference between 5f and 6d is very small in early actinides (Th to U). This leads to more variable oxidation states in actinides than in lanthanides.
General Properties of Actinides
- Radioactivity: All actinides are radioactive. Pm is the only radioactive lanthanide. Th, U, and Pu are naturally abundant; others are artificially made.
- Physical state: Silvery white metals; reactive — tarnish in air forming oxide coatings.
- Magnetic: Paramagnetic due to unpaired 5f electrons; but orbital contribution to magnetism is NOT quenched (unlike lanthanides' partial quenching). Magnetic moments deviate from spin-only values.
- Oxidation states: Show wide range — +3 to +7 (and even higher in some theoretical compounds). +3 and +4 most common. U can reach +6 in UO₂²⁺ (uranyl ion), Np can be +7.
- Complex formation: Greater tendency to form complexes than lanthanides due to larger ionic size and 5f orbital participation. Form stable anionic complexes with F⁻ and other hard ligands.
- Colour: Most An³⁺ ions are coloured due to f-f and f-d transitions.
- Actinide contraction: Similar to lanthanide contraction; ionic radius decreases from Ac to Lr, but less smooth due to irregular 5f/6d occupancy.
Nuclear Uses of Key Actinides
U-235: Fissile material for nuclear reactors and weapons
Th-232: Fertile material → can be converted to fissile U-233 (India's thorium programme)
Pu-239: Produced in reactors from U-238, used in bombs
Am-241: Used in smoke detectors
Oxidation States of Key Actinides
Lanthanides vs Actinides
| Property | Lanthanides (4f) | Actinides (5f) |
|---|---|---|
| Filling orbital | 4f (inner) | 5f (inner) |
| Elements | Ce (58) to Lu (71) — 14 elements | Th (90) to Lr (103) — 14 elements |
| Radioactivity | Only Pm is radioactive; others are stable | ALL are radioactive |
| Oxidation states | Mainly +3 (stable); +2 (Eu, Sm, Yb) & +4 (Ce, Pr, Tb) are minor | Wide range: +3, +4, +5, +6, +7 — much more variable; +3/+4 common |
| Ionic radii change | Smooth, steady decrease (lanthanide contraction) | Less regular decrease (actinide contraction) — due to complex 5f/6d filling |
| Magnetic moment | Mostly follows spin-only formula; orbital contribution partially quenched | Orbital contribution NOT quenched; magnetic moments deviate significantly from spin-only values |
| Complex formation | Limited; prefer hard O-donor ligands; ionic bonding | Greater tendency; 5f orbitals participate in covalent bonding; form more stable complexes |
| Colour of ions | Colour due to f-f transitions (Laporte forbidden — pale colours) | More intense colours; f-d transitions also contribute; broader absorption bands |
| Covalent character | Predominantly ionic bonding; 4f orbitals too contracted for effective overlap | Greater covalent character; 5f orbitals extend further from nucleus; participate in bonding |
| Availability | Found in nature (monazite, bastnasite ores); most are naturally occurring | Only Th and U naturally occurring; rest are man-made transuranium elements |
| Electronic configuration | [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s² — 5d rarely occupied | [Rn] 5f¹⁻¹⁴ 6d⁰⁻² 7s² — 6d more commonly occupied (esp. early actinides) |
| Industrial importance | Used in catalysts, alloys (Misch metal), phosphors, lasers, magnets | Primarily nuclear fuel (U, Th, Pu); Am in smoke detectors; limited other uses |
Why Lanthanides are "More Regular"
In lanthanides, the 4f orbitals are deeply buried inside the atom (below the 5s²5p⁶ core). They have almost no interaction with the chemical environment, which is why lanthanides are so similar to each other in chemistry. The 5d and 6s electrons determine all chemical behaviour → predominantly +3 and ionic.
Why Actinides are "More Variable"
In actinides, the 5f orbitals are less contracted and extend further into space. The energy gap between 5f, 6d, and 7s is small (especially in early actinides). This means multiple orbitals participate in bonding, giving rise to a wide range of oxidation states, greater covalent character, and more variable chemistry than lanthanides.
- All actinides are radioactive; only Pm among lanthanides is
- Lanthanides: mainly +3; Actinides: wide range +3 to +7 (covalent bonding through 5f)
- Lanthanide contraction: smooth & regular; Actinide contraction: irregular
- Actinides form more stable and varied complexes than lanthanides
- Misch metal (alloy of Ce + other lanthanides) = used in flint (firestarters) and alloy steel
- U-235 is fissile; U-238 is fertile; Th-232 is fertile (can breed U-233)
- Ce is the most abundant lanthanide; Pm is synthetic / not stable in nature
- Gd³⁺ (f⁷) has the highest magnetic moment among Ln³⁺ ions → used in MRI contrast agents