checkmate

Here is a highly detailed, rigorous checklist for the A Level Chemistry: Chemistry of Transition Elements topic. It has been built using the exact requirements of the syllabus, the specific questions asked in the past papers (2009–2025), the strict nuances demanded by the mark schemes, and the common pitfalls highlighted in the examiner reports.

Inferences on what you might be asked next, based on syllabus trends, are included in bold italics.

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🏆 Transition Elements Ultimate Master Checklist

1. General Physical and Chemical Properties

Definitions & Core Concepts

• Define a transition element: A d-block element which forms one or more stable ions with an incomplete (partially filled) d-subshell/d-orbital.
  • Examiner Nuance: Do not confuse the definition of the element with the reason it forms complexes. The definition must mention the stable ion with an incomplete d-subshell.
• State typical chemical properties of transition elements: Variable oxidation states, act as catalysts, form complex ions, form coloured compounds.
• Explain variable oxidation states: The 3d and 4s sub-shells/orbitals are close/similar in energy.
  • Inference: You may be asked to explain why s-block elements (like Ca) do NOT show variable oxidation states (because the energy gap between their outer s-orbital and the next inner orbital is too large).
• Write electronic configurations: Use [Ar] shorthand. Remember 4s electrons are lost first when forming ions.
  • Pitfall: Fe is [Ar] 3d⁶ 4s². Fe³⁺ is [Ar] 3d⁵. Do not write 4s² 3d³ for Fe³⁺! Cu is [Ar] 3d¹⁰ 4s¹ and Cu²⁺ is [Ar] 3d⁹.

Orbital Shapes

• Define degenerate orbitals: Orbitals that have the same energy.
  • Examiner Nuance: Never define this as “non-degenerate”. Do not say “similar energy”—they must be exactly the same energy.
• Define non-degenerate orbitals: Orbitals that do not have the same energy / have different energies.
• Sketch the orbital: Draw four lobes situated exactly between the x and y axes.
  • Examiner Nuance: If you draw the lobes on the axes, you will lose the mark (that is the orbital).
• Sketch the orbital: Draw two lobes directly on the z-axis with a ring/doughnut shape centered in the xy-plane around the nucleus.
  • Inference: Even though and are explicitly mentioned in the syllabus, you should be able to recognise or sketch all 5 d-orbitals.

Catalysis

• Explain why transition elements behave as catalysts (General):
  1. They have more than one stable oxidation state (variable oxidation states).
  2. They have empty/vacant d-orbitals that are energetically accessible to accept lone pairs/form dative bonds with ligands.
• Describe the mode of action of a heterogeneous catalyst (e.g., Pt in catalytic converters, Fe in Haber process):
  1. Adsorption of reactants onto the catalyst surface.
  2. Bonds within the reactant molecules are weakened.
  3. Desorption of the products.
  • Examiner Nuance: Do NOT say “intermolecular forces are weakened” or “bonds between reactants are weakened”. You must specify that the covalent bonds within the reactant molecules are weakened.
• Provide equations for homogeneous catalysis: e.g., NO₂ catalysing the oxidation of SO₂.
  • Step 1: NO₂ + SO₂ NO + SO₃
  • Step 2: 2NO + O₂ 2NO₂
• Provide equations for homogeneous catalysis (Fe³⁺ or Fe²⁺ with I⁻ and S₂O₈²⁻):
  • Step 1: 2Fe³⁺ + 2I⁻ 2Fe²⁺ + I₂
  • Step 2: 2Fe²⁺ + S₂O₈²⁻ 2Fe³⁺ + 2SO₄²⁻
  • Inference: Be prepared to use values to prove WHY a specific transition metal ion can act as a catalyst (the of the catalyst half-cell must lie BETWEEN the values of the two reactant half-cells).

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2. Reactions and Complex Ion Formation

Ligands & Complexes

• Define a complex ion: A molecule or ion formed by a central metal atom/ion surrounded by/bonded to one or more ligands.
• Define a ligand: A species that contains a lone pair of electrons that forms a dative covalent (coordinate) bond to a central metal atom/ion.
• Define coordination number: The total number of dative/co-ordinate bonds being formed by the metal ion.
  • Examiner Nuance: Do not say “number of ligands”. A bidentate ligand forms two coordinate bonds, so 3 bidentate ligands = a coordination number of 6.
• Categorise ligands by denticity:
  • Monodentate (donates 1 lone pair): H₂O, NH₃, Cl⁻, CN⁻.
  • Bidentate (donates 2 lone pairs): 1,2-diaminoethane (en), ethanedioate (C₂O₄²⁻).
  • Polydentate/Hexadentate (donates >2 lone pairs): EDTA⁴⁻. (Donates 6 lone pairs: 2 from N, 4 from O⁻).
• Explain why transition elements form complex ions: They have vacant d-orbitals that are energetically accessible to accept lone pairs from ligands.

Specific Aqueous Reactions (Memorise these!)

• Copper (Cu²⁺) Reactions:
  • Starting material: [Cu(H₂O)₆]²⁺ (Pale blue solution).
  • Add dropwise NaOH or NH₃: Forms Cu(H₂O)₄(OH)₂ or Cu(OH)₂ (Pale blue precipitate). Acid-base / Deprotonation / Precipitation reaction.
  • Add excess NH₃: Forms [Cu(NH₃)₄(H₂O)₂]²⁺ (Deep/dark blue solution). Ligand exchange reaction.
  • Add excess conc. HCl: Forms [CuCl₄]²⁻ (Yellow solution). Ligand exchange reaction. Note the change in geometry from octahedral to tetrahedral.
• Cobalt (Co²⁺) Reactions:
  • Starting material: [Co(H₂O)₆]²⁺ (Pink solution).
  • Add dropwise NaOH: Forms Co(H₂O)₄(OH)₂ or Co(OH)₂ (Blue/Pink precipitate).
  • Add excess NH₃: Forms [Co(NH₃)₆]²⁺ (Straw/yellow-brown solution).
  • Add excess conc. HCl: Forms [CoCl₄]²⁻ (Blue solution).
  • Examiner Nuance: You MUST balance your equations perfectly, including charges and the correct number of displaced waters/hydroxides. e.g., [Co(H₂O)₆]²⁺ + 2OH⁻ -> Co(H₂O)₄(OH)₂ + 2H₂O.
• Tollens’ Reagent Synthesis (Silver Complexes):
  • Step 1: 2Ag⁺ + 2OH⁻ -> Ag₂O(s) + H₂O (Brown precipitate forms).
  • Step 2: Ag₂O(s) + 4NH₃ + H₂O -> 2[Ag(NH₃)₂]⁺ + 2OH⁻ (Colourless solution forms).
  • Inference: They frequently ask for the bond angles of [Ag(NH₃)₂]⁺. The N-Ag-N bond is 180° (linear), but the H-N-Ag bond angle is 109.5° (tetrahedral around the Nitrogen atom).

Redox Titrations & Calculations

• Manganate(VII) / Ethanedioate (MnO₄⁻ / C₂O₄²⁻):
  • Ratio: 2 MnO₄⁻ reacts with 5 C₂O₄²⁻.
  • Conditions: Acidic, heating required.
  • Eq: 2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ -> 2Mn²⁺ + 10CO₂ + 8H₂O
• Manganate(VII) / Iron(II) (MnO₄⁻ / Fe²⁺):
  • Ratio: 1 MnO₄⁻ reacts with 5 Fe²⁺.
  • Eq: MnO₄⁻ + 5Fe²⁺ + 8H⁺ -> Mn²⁺ + 5Fe³⁺ + 4H₂O
• Copper(II) / Iodide (Cu²⁺ / I⁻):
  • Reaction 1: 2Cu²⁺ + 4I⁻ -> 2CuI(s) + I₂(aq) (White ppt of CuI in brown I₂ solution).
  • Reaction 2 (Titration): I₂ + 2S₂O₃²⁻ -> 2I⁻ + S₄O₆²⁻
  • Overall mole ratio: 1 mol S₂O₃²⁻ = 1 mol Cu²⁺.
  • Examiner Nuance: Pay incredibly close attention to dilution factors in these questions (e.g., if you titrate 25cm³ out of a 250cm³ volumetric flask, you MUST multiply your final moles by 10).

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3. Colour of Complexes

• Explain the origin of colour in transition metal complexes:
  1. The d-orbitals split into two different energy levels (non-degenerate sets) due to repulsion from ligand electrons.
  2. Electrons absorb energy (a photon) and are promoted/excited from a lower d-orbital to a higher d-orbital (d-d transition).
  3. A specific frequency/wavelength of visible light is absorbed.
  4. The colour observed is the complementary colour (the light that is transmitted/reflected).
• Explain why different complexes of the same metal have different colours:
  • Different ligands cause a different energy gap () between the split d-orbitals.
  • Therefore, a different frequency/wavelength of visible light is absorbed.
  • Examiner Nuance: Do not just say ” changes”. You must state “they have a different “.
• Explain why Sc³⁺, Ti⁴⁺, Cu⁺, Ag⁺, and Zn²⁺ complexes are colourless:
  • Ions like Cu⁺, Ag⁺, Zn²⁺ have a full d-subshell (d¹⁰).
  • Ions like Sc³⁺, Ti⁴⁺ have an empty d-subshell (d⁰).
  • Therefore, d-d transitions are not possible / no d-electrons can be promoted.
  • Examiner Nuance: You must explicitly state both that the subshell is full/empty AND that d-d transitions cannot occur.

Splitting Patterns

• Octahedral splitting: 2 orbitals at higher energy, 3 orbitals at lower energy.
• Tetrahedral splitting: 3 orbitals at higher energy, 2 orbitals at lower energy.

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4. Stereoisomerism in Complexes

• Recognise Geometrical (cis/trans) isomerism:
  • Square planar: e.g., [Pt(NH₃)₂Cl₂] (cis-platin and trans-platin).
  • Octahedral (with monodentate ligands): e.g., [Co(NH₃)₄(H₂O)₂]²⁺.
  • Octahedral (with bidentate ligands): e.g., [Ni(en)₂Cl₂].
• Recognise Optical isomerism:
  • Octahedral with 3 bidentate ligands: e.g., [Cr(en)₃]³⁺ (forms non-superimposable mirror images).
  • Octahedral with 2 bidentate ligands in the cis configuration: e.g., cis-[Ru(en)₂Cl₂]⁺.
  • Inference: Be able to spot that the TRANS isomer of a complex like [Ru(en)₂Cl₂]⁺ does NOT show optical isomerism because it has a plane of symmetry.
• Determine Polarity:
  • Trans isomers are generally non-polar because the dipoles / polar bonds / partial charges cancel out.
  • Cis isomers are generally polar because the dipoles do not cancel out (the molecule is asymmetric).
  • Examiner Nuance: When explaining why a trans isomer is non-polar, specify that the “dipoles cancel” or “partial charges cancel”. Do not just say “bonds cancel”.
• Drawing 3D Isomers:
  • Use wedges (coming out) and dashes (going in) correctly.
  • Examiner Nuance: Ensure your octahedral drawings actually look octahedral. The axial bonds must be 180° to each other (vertical line), and the equatorial bonds must be shown with wedges and dashes crossing the center. Do not draw distorted shapes.

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5. Stability Constants ()

• Define stability constant (): The equilibrium constant for the formation of a complex in a solvent from its constituent ions or molecules.
• Write expressions for :
  • E.g., for [Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O
  • Crucial Rule: Water ([H₂O]) is NEVER included in the expression because it is the solvent and its concentration is effectively constant.
  • Examiner Nuance: Put the charge of the complex inside the outer concentration brackets. e.g., [[Fe(SCN)]²⁺], not [Fe(SCN)]²⁺.
• Interpret values:
  • A higher/larger value means the complex is more stable.
  • Ligand exchange reactions will always favour the formation of the complex with the larger value.
• Perform calculations:
  • Use given equilibrium concentrations to find .
  • Given and initial concentrations, calculate the concentration of a specific ion at equilibrium.
  • Inference: Always check the units of in calculations. You may be asked to deduce the units of for a specific complex formation (e.g., ).