JEE Chemistry The D and F Block Elements Notes Set 02

Position and Electronic Configuration

A transition element has an incomplete d subshell either in the neutral atom or in its common ions. Their general configuration is \( (n-1)d^{1-10}ns^{0-2} \). Zn, Cd and Hg have a full d¹⁰ subshell even in their ions, so they are usually treated as d-block but not true transition elements.
There are four series: 3d (Sc to Zn), 4d (Y to Cd), 5d (La, Hf to Hg) and 6d (Ac onwards). Cr (3d⁵ 4s¹) and Cu (3d¹⁰ 4s¹) are anomalous because half-filled and fully-filled d subshells are extra stable. During ionisation the ns electrons are lost before the (n-1)d electrons.

General Properties

Metallic character: all are hard, lustrous metals with high melting and boiling points, because both ns and (n-1)d electrons take part in metallic bonding. Melting point rises to a maximum near the middle of the series (d⁵) and falls towards Zn, Cd and Hg, which are the lowest (Hg is a liquid).
Atomic size: radii fall slightly from Sc to about Cr and then stay nearly constant up to Cu, because added d electrons shield the outer electrons and cancel the rise in nuclear charge. The 5d elements are almost the same size as the 4d elements because of the lanthanoid contraction, so Zr and Hf (and Nb and Ta) are very hard to separate.
Density increases across a series; Os (22.6 g/cm³) and Ir are the densest elements, Sc the lightest transition metal.
Ionisation energy rises slowly across the series. Zn, Cd and Hg have high values because of the stable d¹⁰ shell.

Oxidation States

Because the (n-1)d and ns energies are close, several electrons can be used in bonding. The number of states is greatest in the middle of the series; Mn shows every state from +2 to +7. The highest state equals the sum of ns and (n-1)d electrons up to Mn (e.g. +7 for Mn) and then falls (Fe +6 at most, Zn only +2).
Lower states are mostly ionic, higher states covalent. In a group, stability of the higher states rises on moving down (Cr(VI) is oxidising but Mo(VI) and W(VI) are stable). Mn²⁺ (d⁵) and Fe³⁺ (d⁵) are particularly stable because of the half-filled subshell.
Highest oxide of Mn is \( \ce{Mn2O7} \) (acidic), while the oxides in lower states are basic; acidity increases with oxidation state.

Magnetic Properties

Substances with unpaired electrons are paramagnetic; with all electrons paired they are diamagnetic. The spin-only magnetic moment is \( \mu=\sqrt{n(n+2)}\ \text{BM} \), where n is the number of unpaired electrons.
Examples: \( \ce{Fe^{3+}} \) (d⁵, n = 5) has 5.92 BM, \( \ce{Cu^{2+}} \) (d⁹, n = 1) has 1.73 BM, \( \ce{Zn^{2+}} \) (d¹⁰) is diamagnetic. Fe, Co and Ni are ferromagnetic.

Colour, Complexes and Catalysis

Colour: ions with partly filled d orbitals absorb visible light to promote an electron between d orbitals (d-d transition) and appear in the complementary colour. d⁰ ions (Sc³⁺, Ti⁴⁺) and d¹⁰ ions (Zn²⁺, Cu⁺) are colourless.
Complex formation: small, highly charged ions with vacant d orbitals bind ligands strongly and form many complexes.
Catalysis: variable oxidation states and the ability to adsorb reactants make them good catalysts, e.g. V₂O₅ in the contact process, Fe in the Haber process, Ni in hydrogenation.
Other features: they form interstitial compounds (hydrides, carbides, nitrides) and alloys such as brass and steel.

Colours of Common Ions

Potassium Dichromate

Preparation: chromite ore is fused with sodium carbonate in air to give sodium chromate, which is acidified to sodium dichromate and converted to the potassium salt with KCl.
\( \ce{4FeCr2O4 + 8Na2CO3 + 7O2 -> 8Na2CrO4 + 2Fe2O3 + 8CO2} \); \( \ce{2Na2CrO4 + 2H^+ -> Na2Cr2O7 + 2Na^+ + H2O} \).
Chromate (yellow) and dichromate (orange) are interconverted by pH: acid shifts the equilibrium to dichromate, alkali to chromate. \( \ce{2CrO4^{2-} + 2H^+ <=> Cr2O7^{2-} + H2O} \).
It is a strong oxidising agent in acid: \( \ce{Cr2O7^{2-} + 14H^+ + 6e^- -> 2Cr^{3+} + 7H2O} \). It oxidises \( \ce{Fe^{2+}} \) to \( \ce{Fe^{3+}} \) (1 mol dichromate to 6 mol Fe²⁺), iodide to iodine, sulphite to sulphate and H₂S to sulphur. It is a primary standard in volumetric analysis.

Potassium Permanganate

Preparation: pyrolusite \( \ce{MnO2} \) is fused with KOH in air to give green potassium manganate, which is oxidised (electrolytically or by chlorine/ozone) to the purple permanganate.
\( \ce{2MnO2 + 4KOH + O2 -> 2K2MnO4 + 2H2O} \); \( \ce{2MnO4^{2-} + Cl2 -> 2MnO4^- + 2Cl^-} \).
Oxidising action depends on the medium: in acid it forms \( \ce{Mn^{2+}} \) (5 electrons), in neutral or weakly alkaline medium \( \ce{MnO2} \) (3 electrons) and in strongly alkaline medium manganate (1 electron).
Acidic medium: \( \ce{MnO4^- + 8H^+ + 5e^- -> Mn^{2+} + 4H2O} \). It oxidises \( \ce{Fe^{2+}} \), oxalate, H₂S, sulphite and iodide. It is a self-indicator (the pink end point) and is used only with dilute H₂SO₄, never HCl, which would be oxidised to chlorine.

Compounds of Other Metals

Iron: Fe²⁺ salts are pale green and easily oxidised; Mohr's salt \( \ce{FeSO4.(NH4)2SO4.6H2O} \) is a stable double salt used as a primary standard. Fe³⁺ gives a blood-red colour with thiocyanate and Prussian blue with ferrocyanide.
Copper: blue vitriol \( \ce{CuSO4.5H2O} \) loses water on heating to give white anhydrous salt. Cu²⁺ with excess ammonia gives the deep-blue \( \ce{[Cu(NH3)4]^{2+}} \). Cu⁺ (d¹⁰) is colourless and disproportionates in water to Cu and Cu²⁺.
Silver: AgNO₃ is made from silver and nitric acid; silver halides darken in light (used in photography), and AgBr is dissolved by hypo (sodium thiosulphate). Ag⁺ forms \( \ce{[Ag(NH3)2]^+} \) (Tollens' reagent).
Zinc and mercury: Zn dissolves in both acids and alkalis, and ZnO is amphoteric. Mercury is the only liquid metal and forms amalgams with most metals (not with Fe).

Inner Transition Elements: Lanthanoids

The 14 elements Ce to Lu (with La) fill the 4f subshell: general configuration \( [Xe]4f^{1-14}5d^{0-1}6s^2 \). The most common oxidation state is +3; Ce also shows +4 (4f⁰) and Eu +2 (4f⁷) and Yb +2 (4f¹⁴), Tb +4 (4f⁷).
Lanthanoid contraction: the ionic radius falls steadily from La³⁺ to Lu³⁺ because 4f electrons shield poorly. It makes the 5d elements nearly the same size as the 4d elements, lowers the basicity of the hydroxides from La(OH)₃ to Lu(OH)₃ and makes the lanthanoids hard to separate (ion exchange is used).
Uses: misch metal (about 95% lanthanoids, 5% Fe) for lighter flints and alloys, and for steel production.

Inner Transition Elements: Actinoids

The 14 elements Th to Lr fill the 5f subshell. Unlike the lanthanoids they show a wide range of oxidation states (+3 to +7 for Np and Pu), because 5f, 6d and 7s energies are close. All are radioactive; those after uranium are man-made. They show an actinoid contraction that is larger than the lanthanoid contraction because 5f electrons shield even less.
Comparison: lanthanoids mostly show only +3, form less complexes and are less basic-variable; actinoids show many states, form complexes more readily and are radioactive.

Worked Examples

Example 1. Calculate the spin-only magnetic moment of \( \ce{Fe^{2+}} \).
Answer:
\( \ce{Fe^{2+}} \) is 3d⁶ with 4 unpaired electrons.
\( \mu=\sqrt{4\times6}=\sqrt{24} \) = 4.90 BM.

Example 2. How many moles of \( \ce{KMnO4} \) are needed to oxidise 1 mol of \( \ce{FeC2O4} \) in acidic medium?
Answer:
Both \( \ce{Fe^{2+}} \) (1 e⁻) and oxalate (2 e⁻) are oxidised, so 3 mol of electrons are lost per mole of \( \ce{FeC2O4} \).
Each permanganate gains 5 electrons.
Moles of \( \ce{KMnO4} \) = 3/5 = 0.6 mol.

Example 3. Why is Mn²⁺ more stable than Mn³⁺ while Fe³⁺ is more stable than Fe²⁺?
Answer:
Mn²⁺ is 3d⁵ (half-filled, extra stable) so a further oxidation is difficult.
Fe³⁺ is 3d⁵ (half-filled) while Fe²⁺ is 3d⁶, so Fe²⁺ readily loses one electron to reach d⁵.