For transition and inner transition ions, ns electrons are removed before (n-1)d or (n-2)f electrons. Applying this rule to the neutral configurations gives the listed ionic configurations.
Cr3+: [Ar]3d3; Pm3+: [Xe]4f4; Cu+: [Ar]3d10; Ce4+: [Xe]; Co2+: [Ar]3d7; Lu2+: [Xe]4f14 5d1; Mn2+: [Ar]3d5; Th4+: [Rn].
Oxidation of Mn2+ to Mn3+ changes 3d5 to 3d4, losing the extra stability of a half-filled d subshell. Oxidation of Fe2+ to Fe3+ changes 3d6 to 3d5, producing the stable half-filled configuration. Hence Mn2+ resists oxidation more than Fe2+.
Mn2+ has a stable half-filled 3d5 configuration, whereas Fe2+ is 3d6 and can be oxidised to the more stable Fe3+ 3d5 state.
In the first half of the 3d series, M2+ ions are formed by removing two 4s electrons. As nuclear charge increases, further removal of an electron from the 3d subshell becomes increasingly difficult. Therefore the +2 state becomes more stable relative to +3 with increasing atomic number across the first half.
The +2 state becomes relatively more stable because the third ionisation enthalpy increases and removal of a third electron becomes less favourable.
Stable half-filled and completely filled d subshells favour some oxidation states. Mn2+ is stable because it is 3d5, and Zn2+ is stable because it is 3d10. Fe3+ is more stable than Fe2+ with respect to oxidation because Fe3+ is 3d5. However, electronic configuration alone is not enough; hydration enthalpy, lattice enthalpy and ionisation enthalpy also affect stability in compounds and solutions.
Electronic configurations strongly influence stability, especially d0, d5 and d10 arrangements, but lattice, hydration and ionisation energies also matter.
In VO3-, V is +5, equal to group 5. In CrO4^2- and Cr2O7^2-, Cr is +6, equal to group 6. In MnO4-, Mn is +7, equal to group 7. These are oxometal anions of the first transition series.
Vanadate, VO3-; chromate, CrO4^2-; dichromate, Cr2O7^2-; and permanganate, MnO4- are common examples.
As nuclear charge increases across the lanthanoid series, added 4f electrons shield poorly. Effective nuclear attraction increases, so size decreases gradually. Consequences include close similarity of 4d and 5d transition elements such as Zr and Hf, difficulty in separating lanthanoids, increasing covalent character and basicity decrease of lanthanoid hydroxides from La(OH)3 to Lu(OH)3.
Lanthanoid contraction is the gradual decrease in atomic and M3+ ionic radii from La to Lu due to poor shielding by 4f electrons.
Transition elements lie between s- and p-block elements and have incomplete d subshells in atoms or common ions. Their characteristic properties arise from partly filled d orbitals. Zn, Cd and Hg have d10 configurations in atoms and common +2 ions, so they are not regarded as transition elements by the strict definition.
They have partly filled d orbitals in atoms or ions and show variable oxidation states, coloured ions, magnetic behaviour, complex formation and catalytic activity. Group 12 elements Zn, Cd and Hg are generally not true transition elements.
The general outer configuration of d-block transition elements is (n-1)d1-10 ns1-2. In non-transition s- and p-block elements, the differentiating electron enters ns or np orbitals rather than an inner d subshell.
Transition elements have progressively filled inner (n-1)d orbitals, whereas non-transition elements fill only outer s or p orbitals in their valence shells.
Most lanthanoids form stable Ln3+ ions. +2 states occur for elements such as Eu and Yb, where f7 or f14 stability is involved. +4 states occur for elements such as Ce and Tb, where f0 or near-stable configurations are favoured.
The principal oxidation state is +3; some lanthanoids also show +2 or +4.
(i) Paramagnetism is due to unpaired d electrons. (ii) Strong metallic bonding involves both ns and (n-1)d electrons, giving high enthalpies of atomisation. (iii) Coloured compounds arise from d-d transitions or charge-transfer transitions in partly filled d subshells. (iv) Catalytic activity is due to variable oxidation states, ability to adsorb reactants and form intermediate complexes.
These properties arise mainly from partly filled d orbitals, strong metallic bonding, d-d transitions and variable oxidation states/surface activity.
Small non-metal atoms can enter interstitial sites of transition-metal lattices without greatly changing the metal structure. Transition metals form such compounds readily because of their close-packed structures and ability to bond with small atoms. Examples include TiC, Fe3H and Mn4N.
Interstitial compounds are formed when small atoms such as H, C or N occupy holes in a metal lattice. They are common for transition metals because these metals have suitable lattice voids and variable bonding ability.
In transition metals, ns and (n-1)d electrons have comparable energies, so successive electrons can participate in bonding. For example, Mn shows +2, +3, +4, +6 and +7. Non-transition p-block elements often show oxidation states differing by two due to inert-pair effects, such as Sn(II)/Sn(IV) or Pb(II)/Pb(IV).
Transition metals usually show many oxidation states differing by one, while non-transition elements often show states differing by two.
Chromite ore FeCr2O4 is fused with sodium carbonate in air to form sodium chromate. Acidification converts chromate to dichromate, and treatment with KCl gives less soluble K2Cr2O7 crystals. The pH equilibrium is 2CrO4^2- + 2H+ ⇌ Cr2O7^2- + H2O. Increasing pH removes H+ and shifts equilibrium to yellow CrO4^2-.
Chromite is oxidised to chromate, converted to dichromate in acid, then crystallised as K2Cr2O7. Increasing pH converts orange dichromate to yellow chromate.
In acid medium, Cr2O7^2- + 14H+ + 6e- → 2Cr3+ + 7H2O. Therefore: (i) Cr2O7^2- + 14H+ + 6I- → 2Cr3+ + 3I2 + 7H2O. (ii) Cr2O7^2- + 14H+ + 6Fe2+ → 2Cr3+ + 6Fe3+ + 7H2O. (iii) Cr2O7^2- + 8H+ + 3H2S → 2Cr3+ + 7H2O + 3S.
In acid medium, dichromate is reduced to Cr3+ and oxidises I- to I2, Fe2+ to Fe3+, and H2S to S.
Pyrolusite MnO2 is fused with KOH in air or oxidising agent to give green K2MnO4, which is electrolytically oxidised or disproportionated to KMnO4. In acid medium: MnO4- + 8H+ + 5Fe2+ → Mn2+ + 5Fe3+ + 4H2O. 2MnO4- + 5SO2 + 2H2O → 2Mn2+ + 5SO4^2- + 4H+. 2MnO4- + 5C2O4^2- + 16H+ → 2Mn2+ + 10CO2 + 8H2O.
KMnO4 is prepared from MnO2 via manganate; acidified permanganate oxidises Fe2+ to Fe3+, SO2 to sulphate and oxalic acid to CO2.
A high positive E° for M3+/M2+ means M3+ is easily reduced and therefore less stable. Mn3+/Mn2+ = +1.5 V, so Mn3+ is least stable. Cr3+/Cr2+ = -0.4 V, so Cr3+ is most stable toward reduction. Fe3+/Fe2+ = +0.8 V is intermediate. For M2+/M values, more negative potential means the metal is more readily oxidised. Mn (-1.2 V) and Cr (-0.9 V) oxidise more readily than Fe (-0.4 V); among them Mn is easiest.
(i) Fe3+ is less stable than Cr3+ but more stable than Mn3+ toward reduction to +2. (ii) Iron is oxidised less readily than chromium but more readily than manganese from metal to M2+.
Ions with partly filled d subshells are generally coloured due to d-d transitions. Ti3+ is d1, V3+ d2, Mn2+ d5, Fe3+ d5 and Co2+ d7, so they are coloured. Cu+ is d10 and Sc3+ is d0, so they lack d-d transitions and are colourless.
Coloured: Ti3+, V3+, Mn2+, Fe3+ and Co2+. Colourless: Cu+ and Sc3+.
The +2 ions are generally formed by loss of two 4s electrons. Early elements also show higher states readily, but later elements increasingly stabilise +2 because removal of additional d electrons is harder. Mn2+ has half-filled d5 and Zn2+ has filled d10, giving special stability.
The +2 state is common across the first transition series and becomes relatively more stable toward the later elements; Mn2+ and Zn2+ are especially stable due to d5 and d10 configurations.
Lanthanoids fill 4f orbitals and commonly have [Xe]4fn5d0/16s2 configurations; actinoids fill 5f orbitals with more irregular 5f/6d/7s occupancies. Both show contraction, but actinoid contraction is greater. Lanthanoids mainly show +3 with limited +2 and +4 states, whereas actinoids show a wider range such as +3 to +7, especially early members. Actinoids are generally more reactive and more complex because 5f electrons participate in bonding more than 4f electrons.
Both involve f-orbital filling and contraction, but actinoids show more variable oxidation states and greater chemical reactivity.
Cr2+ is d4 and acts as a reducing agent because oxidation to Cr3+ gives d3, which is stable in octahedral fields. Mn3+ is d4 and acts as an oxidising agent because reduction to Mn2+ gives stable d5. Co2+ is stable in water, but ligands such as NH3 stabilise low-spin Co3+ complexes, making oxidation easier. d1 ions are often unstable because they can change oxidation state to attain d0 or more stabilised configurations.
(i) Cr2+ is oxidised to stable Cr3+ d3, while Mn3+ is reduced to stable Mn2+ d5. (ii) Complexation stabilises Co3+. (iii) d1 ions tend to lose or gain electrons to reach more stable configurations.
In disproportionation, an element in one oxidation state changes into two different oxidation states. Examples: 2Cu+(aq) → Cu2+(aq) + Cu(s). Also, 3MnO4^2- + 4H+ → 2MnO4- + MnO2 + 2H2O.
Disproportionation is simultaneous oxidation and reduction of the same species.
Copper frequently shows +1 because Cu+ has a stable 3d10 electronic configuration. This filled d subshell stabilises the +1 oxidation state in many compounds, especially insoluble or complex compounds.
Copper.
Mn3+ is 3d4, Cr3+ is 3d3, V3+ is 3d2 and Ti3+ is 3d1. In gaseous free ions, Hund's rule gives 4, 3, 2 and 1 unpaired electrons respectively. Cr3+ is especially stable in aqueous solution because d3 has high stabilisation in octahedral ligand fields.
Mn3+: 4; Cr3+: 3; V3+: 2; Ti3+: 1. Cr3+ is the most stable in aqueous solution.
(i) Low oxidation state oxides such as MnO are basic, while high oxidation state oxides such as Mn2O7 or CrO3 are acidic because metal-nonmetal character increases with oxidation state. (ii) Oxygen and fluorine are highly electronegative and small, so they stabilise high oxidation states, e.g. VF5 and CrO3. (iii) Oxoanions such as MnO4- and CrO4^2- stabilise high oxidation states through strong M-O multiple-bond character and charge delocalisation.
Increasing oxidation state increases covalent and acidic character; oxygen and fluorine stabilise high oxidation states; oxoanions stabilise very high states through M-O bonding.
(i) FeCr2O4 is fused with Na2CO3 in air to form Na2CrO4. Acidification gives Na2Cr2O7, and KCl converts it to K2Cr2O7 by crystallisation. (ii) MnO2 is fused with KOH and an oxidising agent or air to form K2MnO4. The manganate is oxidised electrolytically or disproportionated to KMnO4.
Chromite is converted to chromate, then dichromate, then K2Cr2O7. Pyrolusite is converted to manganate, then permanganate.
Mischmetall contains lanthanoid metals, mainly Ce, La, Nd and Pr with iron and small amounts of other elements. It is used in lighter flints, tracer bullets, shells and as a reducing/deoxidising additive in metallurgy.
Alloys are homogeneous mixtures of metals or metals with non-metals. Mischmetall is an important lanthanoid-containing alloy.
Inner transition elements are lanthanoids and actinoids in which 4f or 5f orbitals are progressively filled. Atomic number 59 is Pr, 95 is Am and 102 is No, all f-block elements. 29 is Cu, 74 is W and 104 is Rf, which are d-block elements.
Inner transition elements are f-block elements. Among the given atomic numbers, 59, 95 and 102 are inner transition elements.
Lanthanoids predominantly show +3 oxidation state, with limited +2 and +4 examples. Actinoids, especially early members, show many states because 5f, 6d and 7s levels are close in energy. For example, Th shows +4, Pa shows +5, U shows +3 to +6, Np and Pu can show +3 to +7. This makes actinoid chemistry less regular.
Actinoids show a wider and less regular range of oxidation states than lanthanoids.
The actinoid series ends at lawrencium, atomic number 103. With the 5f subshell complete, the configuration is represented as [Rn]5f14 6d1 7s2. By losing three outer electrons, it forms Lr3+, so +3 is the expected oxidation state.
Lawrencium, Lr, is the last actinoid. Its configuration is commonly written [Rn]5f14 6d1 7s2; its common oxidation state is +3.
Cerium is [Xe]4f1 5d1 6s2. Removing three electrons gives Ce3+ = [Xe]4f1. It has one unpaired electron. Spin-only magnetic moment μ = sqrt(n(n + 2)) = sqrt(1 x 3) = 1.73 BM.
Ce3+ = [Xe]4f1; magnetic moment = 1.73 BM.
Lanthanoids involve filling of 4f orbitals, which are deeply buried; actinoids involve 5f orbitals, which are less shielded and can participate in bonding. Lanthanoids mainly show +3, while actinoids show +3 along with +4, +5, +6 and +7 in early members. Actinoids are generally more reactive, radioactive and form more complex compounds because 5f, 6d and 7s energies are comparable.
Actinoids have more irregular configurations, more oxidation states and greater reactivity than lanthanoids.
All series have general (n-1)d and ns valence configurations with exceptions. Second and third series transition metals show higher oxidation states more readily and often form stronger metal-metal bonds. Ionisation enthalpies are generally higher for 5d elements than expected. Atomic size increases from 3d to 4d, but 4d and 5d congeners are very similar because lanthanoid contraction offsets the expected increase in size.
The heavier congeners have similar but often more regular configurations, higher common oxidation states, generally higher ionisation enthalpies and very similar 4d/5d sizes due to lanthanoid contraction.
Remove 4s electrons before 3d electrons. In weak-field aqua complexes, electrons occupy octahedral d orbitals according to high-spin filling. Thus d2 and d3 occupy t2g orbitals singly; d5 has five unpaired electrons; d6 high spin is t2g4 eg2; d7 is t2g5 eg2; d8 is t2g6 eg2; d9 is t2g6 eg3.
Ti2+ d2; V2+ d3; Cr3+ d3; Mn2+ d5; Fe2+ d6; Fe3+ d5; Co2+ d7; Ni2+ d8; Cu2+ d9. Hydrated ions are generally high spin.
3d elements generally have lower atomisation enthalpies, fewer metal-metal bonds and lower maximum oxidation states than many 4d/5d elements. Heavier transition elements have more diffuse d orbitals, stronger metal-metal bonding and greater tendency to show high oxidation states. Lanthanoid contraction also makes 4d and 5d congeners unusually similar to each other but distinct from 3d congeners.
The first transition series differs because 3d orbitals are smaller and less diffuse than 4d and 5d orbitals.