Class 12 Chemistry · Chapter 4 Notesd and f Block Elements

The d block occupies the large middle section of the periodic table, between the s block and the p block, and consists of groups 3 to 12. In these elements the d orbitals of the penultimate shell are progressively filled, giving four series: 3d (Sc to Zn), 4d (Y to Cd), 5d (La and Hf to Hg) and 6d (Ac and Rf to Cn). The general outer configuration is (n-1)d1-10 ns1-2, but there are exceptions because the energy gap between the (n-1)d and ns orbitals is very small and half-filled or completely filled orbitals are more stable. Chromium is 3d5 4s1 instead of 3d4 4s2, copper is 3d10 4s1 instead of 3d9 4s2, and palladium is 4d10 5s0. Zinc, cadmium, mercury and copernicium have a completely filled (n-1)d10 ns2 configuration in the ground state and in their common oxidation states, so they are not regarded as transition elements.

• Transition metalsmetals with an incomplete d subshell in the neutral atom or in their ions
• 3d seriesSc (Z = 21) to Zn (Z = 30)
• 4d seriesY (Z = 39) to Cd (Z = 48)
• 5d seriesLa (Z = 57) and Hf (Z = 72) to Hg (Z = 80)
• 6d seriesAc (Z = 89) and Rf (Z = 104) to Cn (Z = 112)

Transition elements show typical metallic properties such as high tensile strength, ductility, malleability, high thermal and electrical conductivity and metallic lustre. Except for Zn, Cd, Hg and Mn, they have one or more typical metallic structures at normal temperatures. Their melting and boiling points are high because (n-1)d electrons in addition to ns electrons take part in metallic bonding; in any row the melting point rises to a maximum at d5 and then falls. Enthalpies of atomisation are also high, with maxima near the middle of each series. Atomic and ionic radii decrease only slightly across a series because d electrons shield the outer electrons rather poorly. The 4d and 5d series have almost the same radii due to the lanthanoid contraction, for example Zr is 160 pm and Hf is 159 pm.

• High melting points and high enthalpies of atomisation
• Small and gradual decrease in atomic and ionic radii across a series
• Lanthanoid contraction makes 4d and 5d elements of a group very similar

Ionisation enthalpy increases along each series from left to right because nuclear charge increases while electrons are added to inner d orbitals. The increase is much less steep than in non-transition elements, and successive ionisation enthalpies do not rise as sharply. Breaks in the trend occur where a d5 or d10 configuration is formed, which is why the third ionisation enthalpy of iron is lower than that of manganese. Transition elements show a great variety of oxidation states because the energies of ns and (n-1)d electrons are close. Manganese shows all states from +2 to +7, while scandium shows only +3 and zinc only +2. Oxidation states of transition metals often differ by unity, unlike non-transition elements where they differ by two. In groups 4 to 10 the heavier members favour the higher oxidation state, the opposite of the inert pair effect in the p block.

• Maximum oxidation state usually equals the sum of ns and (n-1)d electrons, up to manganese
• Cr(VI) as dichromate in acidic medium is a strong oxidising agent, while MoO3 and WO3 are not
• Low or zero oxidation states occur with ligands having pi-acceptor character, as in Ni(CO)4 and Fe(CO)5

The standard electrode potential E°(M2+/M) becomes less negative across the 3d series because the sum of the first and second ionisation enthalpies increases. Copper has a positive value of +0.34 V, which is why it cannot liberate hydrogen from acids; only oxidising acids such as nitric acid and hot concentrated sulphuric acid react with it. The values for Mn, Ni and Zn are more negative than the general trend, being related to the stability of the half-filled d5 configuration in Mn2+, the completely filled d10 configuration in Zn2+ and the very negative enthalpy of hydration of Ni2+. For the M3+/M2+ couple, Mn3+ and Co3+ are the strongest oxidising agents in aqueous solution, while Ti2+, V2+ and Cr2+ are strong reducing agents that liberate hydrogen from dilute acids. The highest oxidation states are stabilised by oxygen and fluorine, as in VF5, CrF6, Mn2O7 and the oxoanions.

• E°(M2+/M) for copper is +0.34 V
• Mn3+ and Co3+ are strong oxidising agents in water
• Ti2+, V2+ and Cr2+ are strong reducing agents

Many transition metal ions are paramagnetic because of unpaired electrons. The magnetic moment is calculated by the spin-only formula mu = sqrt(n(n+2)) Bohr magnetons, where n is the number of unpaired electrons; one unpaired electron gives 1.73 BM. Coloured ions arise when an electron is excited between d orbitals and the light absorbed lies in the visible region, so the observed colour is complementary to the light absorbed. Transition metals form many complex compounds because of their small size, high ionic charge and available d orbitals. They also act as catalysts, for example vanadium(V) oxide in the contact process, iron in the Haber process and nickel in catalytic hydrogenation. Interstitial compounds such as TiC and Fe3H are hard, chemically inert and retain metallic conductivity. Alloys such as steel, brass and bronze are readily formed because the metallic radii of the metals are similar.

• Spin-only magnetic momentmu = sqrt(n(n+2)) BM
• Colour is due to d-d excitation in the visible region
• Catalytic activity is due to variable oxidation states and complex formation
• Interstitial compoundshigh melting point, very hard, chemically inert
• Mischmetall is an alloy of lanthanoid metal (~95%) and iron (~5%)

Potassium dichromate is prepared from chromite ore, FeCr2O4, which is fused with sodium or potassium carbonate in free access of air to give sodium chromate. The yellow solution is acidified with sulphuric acid to give sodium dichromate, which is treated with potassium chloride to crystallise orange potassium dichromate. Chromate and dichromate are interconvertible depending on pH: dichromate forms in acid and chromate in alkali. The chromate ion is tetrahedral while the dichromate ion has two tetrahedra sharing one corner with a Cr-O-Cr bond angle of 126 degrees. Acidified dichromate, E° = 1.33 V, oxidises iodides, sulphides, tin(II) and iron(II). Potassium permanganate is prepared by fusion of MnO2 with an alkali metal hydroxide and an oxidising agent like KNO3, giving dark green K2MnO4 which disproportionates in neutral or acidic solution. It forms dark purple crystals and decomposes at 513 K. Acidified permanganate oxidises oxalates, iron(II), nitrites and iodides, and in neutral or faintly alkaline solution it oxidises iodide to iodate and thiosulphate to sulphate.

• 4 FeCr2O4 + 8 Na2CO3 + 7 O2 gives 8 Na2CrO4 + 2 Fe2O3 + 8 CO2
• 2 CrO4 2- + 2 H+ gives Cr2O7 2- + H2O
• Cr2O7 2- + 14 H+ + 6 e- gives 2 Cr3+ + 7 H2O, E° = 1.33 V
• MnO4 - + 8 H+ + 5 e- gives Mn2+ + 4 H2O, E° = +1.52 V

The lanthanoids are the fourteen elements following lanthanum, from Ce to Lu. Their atoms have a common 6s2 configuration with variable occupancy of the 4f level, and all tripositive ions have the form 4f n. The steady decrease in atomic and ionic radii from lanthanum to lutetium is called the lanthanoid contraction, caused by the imperfect shielding of one 4f electron by another. It makes the radii of the third transition series very similar to those of the second, so Zr and Hf occur together in nature and are difficult to separate. The principal oxidation state is +3, but +2 and +4 states occur occasionally because of the extra stability of empty, half-filled and completely filled f subshells. Ce4+ is a strong oxidant with E°(Ce4+/Ce3+) = +1.74 V, Eu2+ is a strong reducing agent, and Tb(IV) is an oxidant. The metals are silvery white and soft, react with water and burn in halogens.

• Lanthanoid contractionregular decrease in atomic and ionic radii across the series
• Ce4+ has a noble gas configuration and is a good analytical reagent
• Eu2+ (f7) and Yb2+ (f14) are reducing agents

The actinoids are the fourteen elements from Th to Lr. All are radioactive, and the later members have very short half-lives and can be prepared only in nanogram quantities. Their electronic configurations involve variable occupancy of the 5f and 6d subshells, with irregularities related to the stability of f0, f7 and f14 occupancies. The 5f orbitals are less buried than 4f orbitals, so 5f electrons participate in bonding to a greater extent. The actinoid contraction is greater from element to element than the lanthanoid contraction because 5f electrons shield poorly. Actinoids show a wider range of oxidation states, from +3 in general up to +7 in neptunium, because the 5f, 6d and 7s levels have comparable energies. They are highly reactive metals, attacked by boiling water and hydrochloric acid, and their magnetic behaviour is more complex than that of the lanthanoids.

• ActinoidsTh (Z = 90) to Lr (Z = 103)
• Maximum oxidation state rises from +4 in Th to +7 in Np and then falls
• 5f electrons are more available for bonding than 4f electrons