Overview
This lecture covers the properties, classification, trends, and key concepts of d-block (transition) elements in the periodic table, especially for class 12 chemistry.
Introduction to d-Block Elements
- d-block elements are found in groups 3 to 12 of the periodic table and are known as transition metals.
- The distinguishing feature is that their last electron enters a d orbital (n-1)d^1–10 ns^1–2.
- Not all d-block elements are transition metals; exceptions include Zn, Cd, and Hg due to fully filled d orbitals.
Electronic Configuration and Classification
- Filling order of orbitals follows the (n + l) rule: lower (n + l) fills first (e.g., 4s before 3d).
- General configuration: (n-1)d^1–10 ns^1–2.
- d-block is divided into four series (first, second, third, fourth), each with similar properties among elements in the same series.
Physical Properties
- d-block elements are typically metallic: high tensile strength, ductility, malleability, lustrous, and good conductors of heat and electricity.
- High melting and boiling points due to strong metallic bonding from multiple unpaired electrons.
- Trends: Melting/boiling points and enthalpy of atomization increase to a maximum in the middle of the series, then decrease.
Atomic Size and Trends
- Atomic size decreases from left to right due to increasing effective nuclear charge (Z_eff), then plateaus or increases due to electron-electron repulsion in d orbitals.
- Down the group, atomic size increases; however, lanthanide contraction results in similar atomic radii for second and third series.
Ionization Enthalpy
- Ionization enthalpy generally increases across the period but shows exceptions (e.g., Zn, Cd, Hg have high values due to stable electron configurations).
- Successive ionization energies are higher; half-filled/full-filled configurations (e.g., Cr, Cu) show specific stability.
Oxidation States
- Most d-block elements exhibit variable oxidation states, except Sc (+3) and Zn (+2).
- Maximum oxidation states are found towards the middle of the series (e.g., Mn: +2 to +7).
- Stability is linked to electronic configurations, particularly half-filled (d^5) and fully filled (d^10) d orbitals.
Standard Electrode Potential
- Trends become less negative across the period, with exceptions (e.g., high negative values for Zn, Mn due to stability).
- Electrode potentials indicate the ease of oxidation/reduction processes.
Chemical Properties and Complex Formation
- d-block elements commonly form colored ions and complexes due to partially filled d orbitals and d-d transitions.
- They readily exhibit variable oxidation states and form complex ions with ligands due to small size, high charges, and availability of empty d orbitals.
Magnetic Properties
- d-block elements may be paramagnetic (unpaired electrons, attracted to magnetic fields) or diamagnetic (paired electrons, repelled).
- Magnetic moment: √[n(n+2)] (n = number of unpaired electrons).
Catalytic Properties
- Many transition metals act as catalysts due to variable oxidation states and ability to form intermediates.
Interstitial Compounds
- Small atoms (C, N, H) fit into interstitial spaces of metal lattices, forming hard, high-melting compounds with good conductivity.
Key Terms & Definitions
- d-block elements — elements with the last electron entering a d orbital.
- Transition metals — d-block elements with incomplete d orbitals in at least one oxidation state.
- Effective nuclear charge (Z_eff) — net positive charge experienced by outer electrons.
- Ionization enthalpy — energy required to remove an electron.
- Oxidation state — the charge an atom would have after ionic approximation of its bonds.
- Complex ion — a central metal ion bonded to surrounding ligands.
- Lanthanide contraction — decrease in atomic radii across lanthanides due to poor shielding by 4f electrons.
Action Items / Next Steps
- Memorize the first d-block series (Sc to Zn) using a mnemonic.
- Review electronic configuration rules and (n + l) principle.
- Practice writing electronic configurations and predicting properties based on them.
- Prepare for the next lecture on f-block elements.