The d- and f-Block Elements chapter explains what makes transition metals genuinely distinctive — variable oxidation states, coloured compounds, catalytic activity and magnetic behaviour — and every one of these characteristic properties traces back to the same underlying feature: a partially filled d or f subshell.
Unlike s and p block elements, transition metals have d electrons available at similar energy to their outer s electrons, which is exactly why they display multiple stable oxidation states rather than just one. That same partially-filled d subshell allows d-d electronic transitions, which absorb specific wavelengths of visible light and is the direct cause of transition metal compounds' characteristic colours. Lanthanide contraction, covered in the f-block section, is this chapter's most consequential single fact — it explains why second and third row transition metals in the same group have almost identical atomic radii, with effects that ripple outward into several other chapters, including Coordination Compounds.
Key transition metal properties
| Property | Explanation |
|---|---|
| Variable oxidation states | d and s electrons at comparable energy allow multiple stable states |
| Coloured compounds | d-d electronic transitions absorb specific visible wavelengths |
| Catalytic activity | variable oxidation states and available d orbitals provide reaction surfaces/intermediates |
| Paramagnetism | arises from unpaired d electrons; magnetic moment relates directly to their number |
| Magnetic moment formula | μ = √(n(n+2)) BM, where n = number of unpaired electrons |
| Lanthanide contraction | steady decrease in atomic/ionic radii across the lanthanide series |
| Effect of lanthanide contraction | near-identical radii of second and third row transition metals in the same group |
| Interstitial compounds | small atoms like H, C, N occupy voids in the metal lattice |
Topics covered
- General characteristics of d-block elements — electronic configuration, and the defining feature of a partially filled d subshell
- Variable oxidation states — and why transition metals, unlike s and p block elements, so readily display several stable states
- Colour of transition metal compounds — the d-d transition explanation, and why compounds with a fully empty or fully filled d subshell are typically colourless
- Magnetic properties — paramagnetism arising from unpaired electrons, and calculating magnetic moment
- Catalytic properties and alloy formation — why transition metals are so widely used industrially as catalysts
- Standard electrode potentials of the first transition series — and the general trend, with specific important exceptions
- Lanthanides — electronic configuration, and lanthanide contraction with its consequences
- Actinides — a brief comparison with lanthanides, including their greater tendency toward variable oxidation states
Why this chapter matters
In NEET and JEE, questions on the colour and magnetic behaviour of specific transition metal ions, and oxidation state identification for common transition metal compounds, are consistently and heavily tested. Lanthanide contraction and its downstream consequences are a reliable conceptual favourite, often connected directly to questions from the following Coordination Compounds chapter.
For Class 12 boards, expect a question explaining the colour or magnetic moment of a specific transition metal ion and a conceptual question on lanthanide contraction and its effects.
Mistakes students make in this chapter
- Assuming every transition metal compound is coloured. Compounds with a completely empty (d⁰) or completely filled (d¹⁰) d subshell are typically colourless, since d-d transitions specifically require a partially filled subshell to be possible at all — Sc³⁺ (d⁰) and Zn²⁺ (d¹⁰) are classic colourless examples.
- Forgetting to calculate the number of unpaired electrons correctly before applying the magnetic moment formula. This requires correctly writing the ion's electronic configuration first (not the neutral atom's), since oxidation state changes which electrons remain.
- Confusing lanthanide contraction's cause with its effect. The cause is poor shielding by f electrons as they're added across the lanthanide series; the effect is a steady decrease in atomic and ionic radii, which then causes second and third row transition metals in the same group to have almost identical sizes.
- Assuming the highest oxidation state of every transition metal is always +2 or +3. Transition metals show a wide range of oxidation states, and the maximum possible oxidation state for early transition metals can go as high as the total number of s and d electrons combined, such as +7 for manganese.
- Believing all f-block elements behave identically to lanthanides. Actinides show a considerably greater tendency toward variable oxidation states than lanthanides do, due to closer energy levels among their 5f, 6d and 7s orbitals.
Worked example: spin-only magnetic moments of three common ions
- Write the neutral atom configurations first. Note chromium-style anomalies affect Cr and Cu, so copper is 3d¹⁰ 4s¹:Mn : [Ar] 3d⁵ 4s² Fe : [Ar] 3d⁶ 4s² Cu : [Ar] 3d¹⁰ 4s¹
- Remove electrons in the right order — this is the trap. Although 4s fills before 3d, it is also emptied first when the ion forms, because once 3d is occupied it drops below 4s in energy:4s electrons leave before 3d electrons
- Write the ion configurations. Each M²⁺ loses both 4s electrons where available:Mn²⁺ : 3d⁵ Fe²⁺ : 3d⁶ Cu²⁺ : 3d⁹
- Count unpaired electrons using Hund's rule in the high-spin arrangement — fill all five d orbitals singly before pairing:Mn²⁺ (d⁵) : 5 unpairedFe²⁺ (d⁶) : 4 unpairedCu²⁺ (d⁹) : 1 unpaired
- Apply the spin-only formula, where n is the number of unpaired electrons and BM is the Bohr magneton:μ = √[n(n+2)] BMMn²⁺ : √35 = 5.92 Fe²⁺ : √24 = 4.90 Cu²⁺ : √3 = 1.73
What the notes contain
The PDF explains the d-d transition mechanism behind transition metal colour with several worked examples, derives magnetic moment calculations from electronic configuration for a range of ions, and covers lanthanide contraction with its specific downstream consequences for transition metal chemistry explained in detail.
How to use this: build the concept from these notes first, then do one quick NCERT pass before the exam for exact wording and diagram labels, and spend whatever time is left on previous year questions.
Questions
Why are transition metal compounds often coloured while main group compounds usually aren't?
Transition metal ions with a partially filled d subshell can undergo d-d electronic transitions, absorbing a specific wavelength of visible light and transmitting the complementary colour, which is what we see. Main group compounds and transition metal ions with completely empty or completely filled d subshells lack this partially-filled requirement and are typically colourless — the PDF explains this with several worked examples.
How do I calculate the magnetic moment of a transition metal ion?
First write the ion's correct electronic configuration (accounting for its oxidation state), count the number of unpaired electrons, then apply μ = √(n(n+2)) in Bohr magnetons. The PDF works through this method for several common ions.
What exactly causes lanthanide contraction?
As you move across the lanthanide series, electrons are added to the inner 4f subshell, which shields the outer electrons from increasing nuclear charge only poorly. This poor shielding means effective nuclear charge experienced by outer electrons steadily increases, causing atomic and ionic radii to steadily contract across the series.
Why do zirconium and hafnium have almost identical atomic radii despite being in different periods?
This is a direct consequence of lanthanide contraction — the contraction that occurs across the lanthanide series (which sits between them in the periodic table) almost exactly cancels out the expected radius increase from adding an entire additional electron shell, leaving these two group members with nearly identical sizes and very similar chemistry.
All other chapters
Every chapter of Class 11 and Class 12 Physics, Chemistry and Biology — each with its own notes and PDF.