Structure 3.1.8 – Transition Elements

What Are Transition Elements?

Transition elements are found in the d-block of the periodic table, typically from Groups 3 to 12. According to the IUPAC definition, a transition element is an element that forms at least one stable ion with an incomplete d-subshell.

Electron Configuration and d-Shell

• Transition elements have the general electronic configuration: \( (n-1)d^{1-9}ns^2 \) or \( (n-1)d^{1-10}ns^1 \).
• Their distinguishing feature is the presence of an incomplete d-subshell in at least one of their oxidation states.
• For example, iron (Fe) has the ground state configuration: \( [\text{Ar}]\,3d^6\,4s^2 \), and its ions Fe²⁺ and Fe³⁺ have configurations with incomplete 3d sublevels.

Definition of a Transition Element

• To qualify as a transition element, the element must form an ion in which the d-subshell is not full (i.e., not d¹⁰).
• Examples:
  • Iron (Fe): Fe²⁺ → 3d⁶, Fe³⁺ → 3d⁵ → both are transition ions.
  • Copper (Cu): Cu⁺ → 3d¹⁰ → not considered a transition ion, but Cu²⁺ → 3d⁹ → is a transition ion.
• Zinc (Zn): Not a transition element. Although it’s in the d-block, Zn²⁺ has the electron configuration 3d¹⁰ — a complete d-subshell.

Properties of Transition Elements

Transition metals are well-known for exhibiting a range of distinctive chemical and physical properties due to their partially filled d orbitals. Below is a breakdown of each property with explanations and examples.

1. Variable Oxidation States

• Transition metals can form multiple stable ions by losing different numbers of d and s electrons.
• This leads to a variety of oxidation states that depend on the specific compound and ligands involved.
• Examples:
  • Iron forms Fe²⁺ and Fe³⁺
  • Manganese forms Mn²⁺ to Mn⁷⁺
  • Chromium forms Cr²⁺, Cr³⁺, and Cr⁶⁺

2. High Melting Points

• Transition metals generally have high melting points due to strong metallic bonding.
• The presence of delocalised d electrons enhances metallic bonding strength across the lattice.
• Example: Iron (Fe) has a melting point of 1538°C.

3. Magnetic Properties

• These arise from unpaired electrons in d orbitals.
• The more unpaired electrons, the stronger the magnetic moment.
• Magnetic behaviour depends on electron configuration:
  • Fe, Co, Ni show ferromagnetism due to domain alignment.
  • Paramagnetism (weak attraction) results from unpaired electrons (not assessed in IB).
• Example: Fe (3d⁶) has four unpaired electrons → strongly magnetic.

4. Catalytic Properties

• Transition metals act as excellent catalysts due to:
  • • Variable oxidation states — allow them to gain/lose electrons easily in redox reactions.
    • Ability to adsorb reactants onto their surface, weakening bonds and lowering activation energy.
• Examples:
  • • Iron in the Haber Process: \( \text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3 \)
    • Vanadium(V) oxide in the Contact Process: \( \text{SO}_2 + \tfrac{1}{2}\text{O}_2 \rightarrow \text{SO}_3 \)
    • Nickel in hydrogenation of alkenes

Types of Catalysis

• Heterogeneous: Catalyst is in a different phase from the reactants.
  • Example: Fe in the Haber Process for ammonia synthesis.
• Homogeneous: Catalyst is in the same phase as the reactants.
  • Example: Fe²⁺ in redox reactions in biological systems (e.g., enzymes).

Catalytic Converters

• Use platinum and rhodium metals to convert toxic gases (CO, NOₓ, hydrocarbons) into less harmful products.
• Structured on a ceramic honeycomb to maximise surface area and reduce cost.

Biological Catalysts

• Transition metals like Fe play a role in enzymes and oxygen transport.
• Example: Haemoglobin contains an Fe²⁺ ion in a porphyrin ring and binds O₂ reversibly. Each haemoglobin molecule can transport 4 oxygen molecules via coordinate bonding.

5. Formation of Coloured Compounds

• Transition metal ions absorb certain wavelengths of visible light.
• This occurs due to d–d transitions — electrons are excited between split d orbitals in the presence of ligands.
• The complementary colour of the absorbed light is observed.
• Colour depends on:
  • Identity of the metal
  • Oxidation state
  • Type of ligand
• Examples:
  • \( \text{Cu}^{2+} \) → Blue solution
  • \( \text{Fe}^{3+} \) → Yellow-brown solution
  • \( \text{Cr}_2\text{O}_7^{2-} \) → Orange
  • \( \text{MnO}_4^- \) → Purple

6. Formation of Complex Ions with Ligands

• Transition metal ions form complex ions by accepting lone pairs from ligands.
• Ligands form coordinate covalent bonds with the central metal ion.
• Complex ion properties depend on:
  • Coordination number (number of ligand bonds)
  • Ligand type (e.g., H₂O, NH₃, Cl⁻, CN⁻)
• Examples:
  • \([ \text{Cu(H}_2\text{O)}_6 ]^{2+}\) — Blue octahedral complex
  • \([ \text{Cr(NH}_3)_6 ]^{3+}\) — Purple octahedral complex
  • \([ \text{Fe(CN)}_6 ]^{3-} \) — Yellow complex

Summary Table: Recognizing Transition Element Properties