Schooling · Class 12 · Chemistry · The d-and f-Block Elements · Chapter on a page
CBSE Class 12 · Chemistry · NCERT Chemistry-I
The d-and f-Block Elements: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 27 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
Fact-checked on 21 Oct 2026: an AI-assisted check of this note against cited sources; the errors it found were corrected that day. Some errors may remain.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- Definition of transition elements
- Variable oxidation states
- Colour of compounds
- Catalytic activity
- Formation of complex ions
- Lanthanoid contraction
- Actinoids
- Formulas and facts
- General electronic configuration of…
- Exceptional configurations
- Oxidation state trend (first series)
- Standard electrode potential
- Dichromate ion in acid
- Permanganate ion in acid
- Lanthanoid configuration
- Worked examples
- Writing electronic configuration and…
- Balancing a redox equation involving…
- Explaining lanthanoid contraction
- Watch out for
- Assuming Zn, Cd and Hg are…
- Removing 3d electrons before 4s when…
- Thinking all transition-metal…
- Confusing lanthanoids with actinoids…
- Forgetting to balance oxygen with…
What this chapter is about
The d-block and f-block elements occupy the central and lower portions of the periodic table. The d-block includes elements where the last electron enters a d-orbital (groups 3–12), while the f-block contains the lanthanoids and actinoids where the last electron enters an f-orbital. The d-block elements are commonly called transition elements because they form a bridge between the highly reactive s-block metals and the less metallic p-block elements.
Key ideas
- Definition of transition elements: Elements whose atoms or common ions have incompletely filled d-orbitals. Zinc, cadmium and mercury are d-block but not true transition elements because their d-orbitals are completely filled in both atomic and common ionic states.
- Variable oxidation states: Transition metals show multiple oxidation states because the (n−1)d and ns electrons have similar energies and can both participate in bonding. For example, iron shows +2 and +3, manganese shows +2 to +7.
- Colour of compounds: Incompletely filled d-orbitals allow d–d transitions when visible light is absorbed. The colour seen is complementary to the wavelength absorbed. Compounds of Zn²⁺ (d¹⁰) are colourless.
- Catalytic activity: Transition metals act as catalysts because they can adopt multiple oxidation states and provide a surface for reactant adsorption (e.g., iron in the Haber process, vanadium(V) oxide in the Contact process).
- Formation of complex ions: Small size and high charge density allow transition metal ions to accept electron pairs from ligands, forming coordination compounds such as [Fe(CN)₆]⁴⁻.
- Lanthanoid contraction: As the 4f orbitals fill across the lanthanoid series, the poor shielding by f-electrons causes a steady decrease in atomic and ionic radii. This makes the sizes of 4d and 5d series elements (e.g., Zr and Hf) almost identical.
- Actinoids: These 5f elements are all radioactive. They show more variable oxidation states than lanthanoids because the 5f, 6d and 7s energy levels are closer together.
Formulas and facts to remember
- General electronic configuration of d-block: (n−1)d¹⁻¹⁰ ns¹⁻². For the first transition series (Sc to Zn), n = 4.
- Exceptional configurations: Chromium is [Ar] 3d⁵ 4s¹ and copper is [Ar] 3d¹⁰ 4s¹ owing to the extra stability of half-filled and fully filled d-orbitals.
- Oxidation state trend (first series): Maximum oxidation state increases from Sc (+3) to Mn (+7), then decreases towards Zn (+2). The +2 state arises from loss of two 4s electrons.
- Standard electrode potential: E° values become less negative across the series, meaning later elements are weaker reducing agents. Copper has a positive E° for Cu²⁺/Cu, so it does not liberate H₂ from dilute acids.
- Dichromate ion in acid: Cr₂O₇²⁻ + 14 H⁺ + 6 e⁻ → 2 Cr³⁺ + 7 H₂O. Orange dichromate is reduced to green Cr³⁺.
- Permanganate ion in acid: MnO₄⁻ + 8 H⁺ + 5 e⁻ → Mn²⁺ + 4 H₂O. Purple permanganate becomes nearly colourless Mn²⁺.
- Lanthanoid configuration: [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s². The +3 state is most stable because removal of two 6s and one 4f or 5d electron gives a stable configuration.
- Actinoid configuration: [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s². Uranium commonly shows +6 (as in UO₂²⁺).
Common mistakes
- Assuming Zn, Cd and Hg are transition elements → They are d-block but not transition elements because their d-orbitals are fully filled in both atom and common ion.
- Removing 3d electrons before 4s when forming cations → Always remove 4s electrons first; they are higher in energy in the cation.
- Thinking all transition-metal compounds are coloured → Compounds with d⁰ or d¹⁰ configuration (e.g., Sc³⁺, Zn²⁺) are colourless because no d–d transition is possible.
- Confusing lanthanoids with actinoids regarding radioactivity → All actinoids are radioactive; most lanthanoids are not.
- Forgetting to balance oxygen with water and hydrogen with H⁺ in acidic-medium redox equations → Use the ion-electron method systematically.
Quick revision
- Transition element: incomplete d-orbitals in atom or common ion.
- Variable oxidation states arise because (n−1)d and ns electrons have similar energies.
- Colour results from d–d electronic transitions; d⁰ and d¹⁰ ions are colourless.
- Lanthanoid contraction makes 4d and 5d congeners nearly equal in size.
- KMnO₄ and K₂Cr₂O₇ are strong oxidising agents; learn their half-reactions in acid.
- Actinoids are all radioactive and show wider range of oxidation states than lanthanoids.