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Must-know facts
20 facts
- 1d-block = groups 3-12; transition metal = incomplete d subshell in the atom or a common ion.
- 2Zn, Cd, Hg (and Cn) are (n−1)d¹⁰ ns² and are not transition elements; Sc (3d¹) is, and Ag is (Ag²⁺ is 4d⁹).
- 3Cr = [Ar] 3d⁵ 4s¹; Cu = [Ar] 3d¹⁰ 4s¹; Pd = [Kr] 4d¹⁰ 5s⁰.
- 4Ions lose ns electrons before (n−1)d: Fe²⁺ = 3d⁶, Fe³⁺ = 3d⁵, Mn²⁺ = 3d⁵, Cu²⁺ = 3d⁹.
- 5Melting points peak near d⁵; enthalpy of atomisation peaks mid-series; Zn has the lowest (126 kJ mol⁻¹).
- 6Lanthanoid contraction makes Zr (160 pm) and Hf (159 pm) almost the same size.
- 7Mn shows +2 to +7, the most states in the 3d row; Sc shows only +3; Zn only +2.
- 8Heavier members of d-groups prefer higher oxidation states: W(VI) and Mo(VI) are more stable than Cr(VI).
- 9Cu is the only first-row metal with positive E°(M²⁺/M) = +0.34 V; it does not release H₂ from dilute acids.
- 10E°(M³⁺/M²⁺): Co +1.97 and Mn +1.57 V (strong oxidants); Cr²⁺, V²⁺, Ti²⁺ are strong reductants.
- 11Oxygen stabilises higher states than fluorine: MnF₄ but Mn₂O₇.
- 12Spin-only μ = √[n(n+2)] BM: 1.73, 2.83, 3.87, 4.90, 5.92 for n = 1 to 5 (Table 4.7 prints 2.84 for n = 2).
- 13d⁰ and d¹⁰ ions (Sc³⁺, Ti⁴⁺, Zn²⁺, Cu⁺) are colourless and diamagnetic.
- 14Aquated colours: Cu²⁺ blue, Ni²⁺ green, Fe²⁺ green, Fe³⁺ yellow, Mn²⁺ pink, Co²⁺ pink, Ti³⁺ purple.
- 15Catalysts: V₂O₅ (Contact), Fe (Haber), Ni (hydrogenation), PdCl₂ (Wacker), TiCl₄ + Al(CH₃)₃ (Ziegler).
- 16Chromate (yellow) ⇌ dichromate (orange) with pH; Cr is +6 in both; Cr-O-Cr angle 126°.
- 17Acid permanganate takes 5 e⁻ (to Mn²⁺); neutral or faintly alkaline takes 3 e⁻ (to MnO₂); dichromate takes 6 e⁻ per ion.
- 18Ce⁴⁺ is an oxidant (E° = +1.74 V); Eu²⁺ and Yb²⁺ are reductants; common lanthanoid state is +3.
- 19Mischmetall ≈ 95% lanthanoid metal + 5% Fe; used in lighter flints.
- 20Actinoid maximum oxidation state: Th +4, Pa +5, U +6, Np +7; actinoid contraction is larger per element than lanthanoid contraction.
Common traps
Where marks are lost
Writing Fe²⁺ as [Ar] 3d⁴ 4s² by removing the 3d electrons first.
Calling zinc a transition element because it sits in the d-block.
Assuming Cr is 3d⁴ 4s² and Cu 3d⁹ 4s² by the simple filling order.
Expecting Hf to be clearly bigger than Zr because it is one period lower.
Thinking Mn³⁺ and Cr²⁺ behave alike because both are d⁴.
Using 5 electrons for permanganate in every titration.
Acidifying a permanganate titration with HCl.
Thinking chromium changes oxidation state when chromate turns into dichromate.
Assuming every Ln ion is +3, so Ce⁴⁺ and Eu²⁺ are impossible.
Believing the observed moment always equals the spin-only value.
Formulas
6 to know
Transition-element configuration
(n−1)d¹⁻¹⁰ ns¹⁻²
Exceptions: Cr 3d⁵4s¹, Cu 3d¹⁰4s¹, Pd 4d¹⁰5s⁰.
Spin-only magnetic moment
μ = √[n(n + 2)] BM
n = number of unpaired electrons; n = 1 gives 1.73 BM, n = 5 gives 5.92 BM.
Dichromate in acid
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
E° = 1.33 V; 1 Cr₂O₇²⁻ oxidises 6 Fe²⁺.
Permanganate in acid
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
E° = +1.52 V; 1 MnO₄⁻ oxidises 5 Fe²⁺; 2 MnO₄⁻ oxidise 5 C₂O₄²⁻.
Permanganate to MnO₂
MnO₄⁻ + 4H⁺ + 3e⁻ → MnO₂ + 2H₂O
E° = +1.69 V; in neutral or faintly alkaline solution MnO₄⁻ also gives MnO₂ (3 e⁻).
Chromate-dichromate
2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O
Acid pushes right (orange), alkali left (yellow); Cr stays +6.
Key terms
17 terms
- Transition element
- A metal whose atom or common ion has an incomplete d subshell.
- Inner transition element
- An f-block element, in which 4f (lanthanoids) or 5f (actinoids) orbitals fill.
- Enthalpy of atomisation
- Energy needed to turn one mole of the solid metal into free gaseous atoms; a measure of metallic bond strength.
- Lanthanoid contraction
- The steady fall in atomic and ionic size from La to Lu, caused by poor shielding by 4f electrons.
- Exchange energy
- Stabilisation that grows with the number of parallel-spin electron pairs in a degenerate set.
- Disproportionation
- A reaction in which one oxidation state converts into a higher and a lower one at the same time.
- Paramagnetic
- Drawn into a magnetic field, because of unpaired electrons.
- Diamagnetic
- Pushed out of a magnetic field; all electrons paired.
- Ferromagnetic
- Very strongly attracted to a magnet; an extreme form of paramagnetism.
- Bohr magneton (BM)
- The unit of magnetic moment used for atoms and ions.
- Spin-only moment
- Magnetic moment counting electron spin alone, √[n(n+2)] BM.
- Complex compound
- A compound in which a metal ion binds several ions or neutral molecules into one species with its own properties.
- Interstitial compound
- A compound formed when small atoms (H, C, N) sit in the holes of a metal lattice; usually non-stoichiometric.
- Alloy
- A blend of metals; solid solutions form when metallic radii are within about 15%.
- Mischmetall
- An alloy of about 95% lanthanoid metal and 5% iron with traces of S, C, Ca and Al.
- Oxocation
- A cation containing metal and oxygen, such as VO₂⁺, VO²⁺ or TiO²⁺.
- Primary standard
- A pure, stable substance weighed directly to make a solution of exactly known concentration, as K₂Cr₂O₇ is.