Transition Elements
8.4 Transition Elements
Definition and General Position in the Periodic Table
- Transition elements are metals found in the central block of the Periodic Table, typically in Groups 3–12.
- They are defined as elements that have:
- An incomplete d-subshell in at least one of their stable oxidation states.
- Examples include iron (Fe), copper (Cu), chromium (Cr), and manganese (Mn).
- They are sometimes called d-block elements because their outer electrons are being added to the d-subshell.
- Many transition metals are important in industry, chemistry, and biological systems.
(a) High Densities
- Transition elements are denser than Group I and Group II metals.
- High density is due to:
- Small atomic radii for their large nuclear charge.
- Strong metallic bonding due to the presence of delocalised electrons.
- Examples:
- Iron (Fe) ≈ 7.87 g/cm³
- Copper (Cu) ≈ 8.96 g/cm³
- Osmium (Os) ≈ 22.59 g/cm³ (one of the densest substances known).
- Practical implications:
- High density makes them useful in construction, heavy machinery, and tools.
(b) High Melting Points
- Most transition metals have very high melting points compared to s-block metals (e.g., alkali metals).
- Cause:
- Strong metallic bonds due to delocalised electrons in both s and d orbitals.
- Compact crystal lattice structures.
- Examples:
- Tungsten (W) ≈ 3422°C (highest melting point of all metals).
- Iron (Fe) ≈ 1538°C.
- Exceptions:
- Mercury (Hg) has a very low melting point (-38.83°C) due to weak metallic bonding from filled d-orbitals.
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(c) Variable Oxidation Numbers
- Many transition metals can lose different numbers of electrons, forming ions with different charges.
- This is because:
- They can lose both their outer s-electrons and some d-electrons.
- Examples:
- Iron (Fe²⁺ and Fe³⁺)
- Copper (Cu⁺ and Cu²⁺)
- Manganese (Mn²⁺, Mn³⁺, Mn⁴⁺, Mn⁷⁺).
- Importance:
- Variable oxidation states allow transition metals to participate in a wide range of chemical reactions.
- Makes them useful in redox chemistry and as catalysts.
(d) Formation of Coloured Compounds
- Transition metal compounds are often brightly coloured in solid and solution form.
- Colour arises because:
- d-orbital electrons absorb visible light.
- Energy from the light excites an electron from a lower d-orbital to a higher one.
- The remaining light reflected/transmitted gives the compound its colour.
- Examples:
- Copper(II) sulfate (CuSO₄·5H₂O) – blue
- Potassium dichromate (K₂Cr₂O₇) – orange
- Potassium permanganate (KMnO₄) – purple
- Colour can change depending on:
- Oxidation state
- Type of ligand
- Coordination number.
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(e) Catalytic Activity
- Transition elements and their compounds often act as catalysts.
- Reasons:
- Variable oxidation states allow them to provide alternative reaction pathways with lower activation energy.
- They can form complexes with reactants, weakening bonds and increasing reaction speed.
- Examples:
- Iron (Fe) in the Haber process (N₂ + 3H₂ ⇌ 2NH₃).
- Vanadium(V) oxide (V₂O₅) in the Contact process for sulfuric acid production (SO₂ + ½O₂ ⇌ SO₃).
- Nickel (Ni) in hydrogenation of alkenes.
- Manganese dioxide (MnO₂) in the decomposition of hydrogen peroxide (2H₂O₂ → 2H₂O + O₂).
- Environmental significance:
- Use of catalysts reduces energy consumption and industrial costs.
Physical Properties Summary Table
| Property | Explanation | Example Values |
|---|---|---|
| High density | Strong metallic bonding and close-packed structure | Fe ≈ 7.87 g/cm³, Os ≈ 22.59 g/cm³ |
| High melting point | Strong metallic bonds from delocalised s and d electrons | Fe ≈ 1538°C, W ≈ 3422°C |
| Hardness | Strong bonding between atoms | Steel harder than alkali metals |
| Lustrous appearance | Metallic bonding allows free electrons to reflect light | Cu (reddish), Ag (silvery) |
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
Chemical Properties Summary Table
| Property | Explanation | Example Compounds |
|---|---|---|
| Variable oxidation states | Ability to lose different numbers of electrons | Fe²⁺, Fe³⁺, Cu⁺, Cu²⁺ |
| Coloured compounds | d-electron transitions absorb visible light | CuSO₄ (blue), KMnO₄ (purple) |
| Catalysis | Provide alternative reaction pathways | Fe in Haber process, Ni in hydrogenation |
Importance of Transition Metals in Daily Life
- Industrial use: Steel (iron + carbon) for construction, copper wiring for electrical use.
- Medical use: Platinum compounds for cancer treatment (cisplatin).
- Environmental use: Catalytic converters in vehicles (Pt, Pd, Rh) reduce harmful emissions.
- Biological importance:
- Iron in haemoglobin for oxygen transport.
- Zinc in enzymes.
- Copper in respiratory enzymes of some animals.
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course