d-Block Elements: Complete Notes, Trends & Practice Questions
Understand transition elements, electronic configurations, oxidation states, colour, magnetism, catalytic properties and important compounds with quick revision tables and exam-style MCQs.
1. What are d-Block Elements?
The d-block elements are elements in which the differentiating electron enters the (n − 1)d subshell. They occupy Groups 3–12 of the periodic table.
They are found in the middle of the periodic table, between the s-block and p-block.
Transition element: An element whose atom has an incomplete d subshell, or which forms at least one common ion with an incomplete d subshell.
2. Electronic Configuration
The differentiating electron generally enters the (n − 1)d subshell. The 3d series runs from Sc to Zn.
| Element | Symbol | Ground-state configuration (condensed) |
|---|---|---|
| Scandium | Sc | [Ar] 3d¹ 4s² |
| Titanium | Ti | [Ar] 3d² 4s² |
| Vanadium | V | [Ar] 3d³ 4s² |
| Chromium | Cr | [Ar] 3d⁵ 4s¹ |
| Manganese | Mn | [Ar] 3d⁵ 4s² |
| Iron | Fe | [Ar] 3d⁶ 4s² |
| Cobalt | Co | [Ar] 3d⁷ 4s² |
| Nickel | Ni | [Ar] 3d⁸ 4s² |
| Copper | Cu | [Ar] 3d¹⁰ 4s¹ |
| Zinc | Zn | [Ar] 3d¹⁰ 4s² |
Why are Cr and Cu exceptions?
Chromium has a 3d⁵ 4s¹ configuration and copper has a 3d¹⁰ 4s¹ configuration. The close energies of the 3d and 4s subshells make these arrangements favourable.
3. Important Trends Across the 3d Series
Atomic and ionic radii
Across the series, atomic radii generally decrease slightly at first and then change only modestly. Increasing nuclear charge is partly offset by shielding from added d electrons.
Ionisation enthalpy
It generally increases across the series, but the trend is not perfectly regular because d and s subshell energies and electron arrangements vary.
Oxidation states
Transition metals show variable oxidation states because the ns and (n − 1)d electrons can have comparable energies. The number of accessible oxidation states varies across the series.
| Element | Common oxidation states to know | Exam reminder |
|---|---|---|
| Sc | +3 | Sc³⁺ is d⁰ |
| Ti | +2, +3, +4 | +4 is d⁰ |
| V | +2, +3, +4, +5 | +5 is d⁰ |
| Cr | +2, +3, +6 | Cr³⁺ is d³; Cr(VI) compounds are oxidising |
| Mn | +2, +4, +7 | Mn²⁺ is d⁵; permanganate contains Mn(VII) |
| Fe | +2, +3 | Fe²⁺ is d⁶; Fe³⁺ is d⁵ |
| Co | +2, +3 | Many cobalt compounds are coloured |
| Ni | +2 | Ni²⁺ is commonly d⁸ |
| Cu | +1, +2 | Cu⁺ is d¹⁰; Cu²⁺ is d⁹ |
| Zn | +2 | Zn²⁺ is d¹⁰ |
Maximum oxidation state: In the 3d series, the highest oxidation state rises to +7 for Mn, then the stability and accessibility of high oxidation states generally decline. The details depend on the compound and chemical environment.
4. Characteristic Properties of Transition Elements
🎨 Colour
Many ions with partially filled d subshells are coloured because of d–d electronic transitions. Colour depends on oxidation state, ligands and geometry.
🧲 Magnetism
Unpaired electrons usually cause paramagnetism. Species with all electrons paired are diamagnetic.
⚗️ Catalysis
Transition metals and their compounds can catalyse reactions through variable oxidation states or adsorption on a surface.
🔗 Complexes
Small, relatively highly charged metal ions can accept lone pairs from ligands and form coordination compounds.
🛠️ Alloys
Similar atomic sizes and metallic character make many transition metals useful in alloys.
🔥 Interstitial compounds
Small atoms such as H, C and N can occupy spaces in metal lattices, often increasing hardness.
Colour: an important qualification
Ions with d⁰ or d¹⁰ configurations generally do not show d–d transitions. However, some d⁰ compounds (for example, permanganate and dichromate) are intensely coloured due to charge-transfer transitions.
| Species | d-electron count | Typical observation |
|---|---|---|
| Sc³⁺ | d⁰ | Colourless in common aqueous salts |
| Ti³⁺ | d¹ | Violet in many aqueous solutions |
| Cr³⁺ | d³ | Often violet or green, depending on surroundings |
| Mn²⁺ | d⁵ high-spin | Pale pink in aqueous solution |
| Fe²⁺ | d⁶ high-spin in water | Pale green in aqueous solution |
| Cu²⁺ | d⁹ | Blue in many aqueous solutions |
| Zn²⁺ | d¹⁰ | Colourless in common aqueous salts |
| MnO₄⁻ | Mn(VII), d⁰ | Purple due to charge transfer |
| Cr₂O₇²⁻ | Cr(VI), d⁰ | Orange due to charge transfer |
Magnetic moment shortcut
For many high-spin first-row transition-metal ions, the spin-only magnetic moment is estimated by:
Here n is the number of unpaired electrons and BM means Bohr magneton. This approximation neglects orbital contributions and is most useful when the spin-only model applies.
5. Important Compounds: KMnO₄ and K₂Cr₂O₇
Potassium permanganate (KMnO₄)
- Mn oxidation state: +7.
- Permanganate ion, MnO₄⁻, is purple.
- In acidic medium, it is commonly reduced to Mn²⁺.
- In neutral or mildly alkaline medium, it is commonly reduced to MnO₂.
- In strongly alkaline medium, reduction to manganate, MnO₄²⁻, can occur.
Potassium dichromate (K₂Cr₂O₇)
- Cr oxidation state: +6.
- Dichromate ion, Cr₂O₇²⁻, is orange; chromate ion, CrO₄²⁻, is yellow.
- Acidification favours orange dichromate; increasing pH favours yellow chromate.
- In acidic medium, dichromate can be reduced to Cr³⁺.
JEE tip: Memorise the oxidation states, colours, reduction products and electron counts. Balance redox equations carefully in the specified medium.
6. High-Yield Facts & Common Traps
- Zn, Cd and Hg: d-block elements, but not transition elements under the standard definition.
- Cr and Cu: Important ground-state configuration exceptions.
- Cations: Remove ns electrons before (n − 1)d electrons.
- Variable oxidation states: Due to the close energies of ns and (n − 1)d electrons.
- Colour: d⁰/d¹⁰ ions usually lack d–d colour, but charge-transfer colour is possible.
- Magnetism: Count unpaired electrons in the ion or complex, not simply in the neutral atom.
- Catalysis: Do not assume every transition metal catalyses every reaction; activity is reaction-specific.
- Potassium dichromate and permanganate: Both are important oxidising agents in suitable media.
7. Practice Questions for Class 12 & JEE Main
Instructions: Attempt each question before opening its answer. These are original practice questions for revision, not claimed to be verbatim previous-year questions.
1Which of the following is not considered a transition element under the standard definition?
A. Fe B. Cu C. Zn D. Cr
Show answer and explanation
Answer: C. Zn. Zn and Zn²⁺ both have a completely filled 3d¹⁰ subshell, so Zn does not meet the usual transition-element definition.
2The ground-state electronic configuration of chromium is:
A. [Ar] 3d⁴ 4s² B. [Ar] 3d⁵ 4s¹ C. [Ar] 3d⁶ D. [Ar] 3d³ 4s³
Show answer and explanation
Answer: B. Chromium has the configuration [Ar] 3d⁵ 4s¹.
3How many unpaired electrons are present in a free Fe³⁺ ion in its ground state?
A. 1 B. 3 C. 4 D. 5
Show answer and explanation
Answer: D. 5. Fe is [Ar] 3d⁶ 4s². Fe³⁺ loses two 4s electrons and one 3d electron, giving 3d⁵, with five unpaired electrons in the free ion.
4The colour of many transition-metal ions is mainly associated with:
A. Nuclear transitions B. d–d electronic transitions C. Neutron transfer D. Proton emission
Show answer and explanation
Answer: B. Absorption of visible light can promote d electrons between split d energy levels. Charge-transfer transitions can also produce colour.
5Which ion has a d⁰ configuration?
A. Ti³⁺ B. V²⁺ C. Sc³⁺ D. Cu²⁺
Show answer and explanation
Answer: C. Sc³⁺. Sc is [Ar] 3d¹ 4s²; removing three electrons gives Sc³⁺ = [Ar], or d⁰.
6In acidic medium, one MnO₄⁻ ion is reduced to Mn²⁺ by accepting how many electrons?
A. 2 B. 3 C. 5 D. 7
Show answer and explanation
Answer: C. 5. Mn changes from oxidation state +7 in MnO₄⁻ to +2 in Mn²⁺, a gain of five electrons.
7Which statement about transition-metal cations is correct?
A. 3d electrons are always removed before 4s electrons
B. 4s electrons are removed before 3d electrons
C. Only p electrons are removed
D. Ion formation never changes electron configuration
Show answer and explanation
Answer: B. For first-row transition metals, 4s electrons are removed before 3d electrons during common cation formation.
8The colour of the dichromate ion in aqueous solution is typically:
A. Purple B. Orange C. Deep blue D. Colourless
Show answer and explanation
Answer: B. Orange. Cr₂O₇²⁻ is orange, whereas CrO₄²⁻ is yellow.
9Using the spin-only formula, what is the approximate magnetic moment of a species with three unpaired electrons?
A. 1.73 BM B. 2.83 BM C. 3.87 BM D. 5.92 BM
Show answer and explanation
Answer: C. 3.87 BM. μ = √[n(n + 2)] = √[3(5)] = √15 ≈ 3.87 BM.
10Which property helps many transition-metal compounds act as catalysts?
A. A single fixed oxidation state in all compounds
B. Variable oxidation states and/or surface adsorption
C. Complete absence of bonding interactions
D. A completely filled d subshell in every ion
Show answer and explanation
Answer: B. Accessible oxidation-state changes and adsorption can provide alternative reaction pathways, depending on the catalyst and reaction.
11What is the oxidation state of Mn in KMnO₄?
A. +2 B. +4 C. +6 D. +7
Show answer and explanation
Answer: D. +7. Potassium is +1 and four oxygens total −8. For a neutral compound, Mn must be +7.
12Which configuration corresponds to Cu⁺?
A. [Ar] 3d⁹ B. [Ar] 3d¹⁰ C. [Ar] 3d⁸ 4s² D. [Ar] 3d¹⁰ 4s¹
Show answer and explanation
Answer: B. Neutral Cu is [Ar] 3d¹⁰ 4s¹. Removing its 4s electron gives Cu⁺ = [Ar] 3d¹⁰.
13Which statement best explains the variable oxidation states of transition elements?
A. The ns and (n − 1)d electrons have comparable energies
B. All d electrons have zero energy
C. Transition metals contain no valence electrons
D. The nucleus changes charge during ordinary ion formation
Show answer and explanation
Answer: A. The comparable energies of the ns and (n − 1)d electrons allow different numbers of these electrons to participate in bonding.
14How many electrons are accepted when Cr₂O₇²⁻ is reduced to 2Cr³⁺ in acidic medium?
A. 2 B. 3 C. 6 D. 12
Show answer and explanation
Answer: C. 6. Each Cr changes from +6 to +3 and gains 3 electrons; two Cr atoms therefore gain 6 electrons in total.
15Which pair is matched correctly?
A. MnO₄⁻ — orange B. Cr₂O₇²⁻ — yellow
C. CrO₄²⁻ — yellow D. Zn²⁺ — strongly paramagnetic with five unpaired electrons
Show answer and explanation
Answer: C. Chromate is yellow. Permanganate is purple, dichromate is orange, and Zn²⁺ is d¹⁰ and diamagnetic.
8. One-Minute Revision
- Definition: Differentiating electron enters (n − 1)d.
- Exceptions: Cr = 3d⁵ 4s¹; Cu = 3d¹⁰ 4s¹.
- Not transition elements: Zn, Cd and Hg under the standard definition.
- Ionisation: ns electrons are removed before (n − 1)d electrons.
- Colour: Often d–d transitions; charge transfer also matters.
- Magnetic moment: μ = √[n(n + 2)] BM for the spin-only approximation.
- KMnO₄: Purple; Mn is +7; acidic reduction gives Mn²⁺.
- K₂Cr₂O₇: Orange; Cr is +6; acidic reduction gives Cr³⁺.
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