Introduction
The d-block (transition metals) and f-block (lanthanides and actinides) elements have partially filled d or f orbitals, which gives rise to distinctive chemistry not seen in the main-group elements of Chapters 8–9: variable oxidation states, coloured compounds, and magnetic behaviour.
1. General properties of transition elements
Transition metals characteristically have high melting points, are good conductors, and form coloured compounds — all traceable to their partially filled d-orbitals. Their electrons in the (n−1)d and ns subshells are close in energy, which is the underlying reason they show multiple accessible oxidation states, unlike main-group elements that typically show one or two.
2. Variable oxidation states and complex formation
Because d-electrons can be removed with relatively similar amounts of energy, transition metals commonly exist in several oxidation states (e.g., iron as Fe²⁺ or Fe³⁺). They readily form coordination complexes — a central metal ion surrounded by ligands (Lewis bases that donate electron pairs via coordinate bonds, Chapter 8) — with the ligand arrangement and number (coordination number) determining the complex’s geometry (commonly octahedral for coordination number 6, or tetrahedral/square planar for 4).
3. Colour and magnetic behaviour
Colour in transition metal complexes arises from d–d electronic transitions: the surrounding ligands split the d-orbitals into groups of different energy (crystal field splitting), and electrons absorb visible light to jump between these split levels, with the observed colour being the complement of the absorbed wavelength. Magnetic behaviour depends on the number of unpaired d-electrons: complexes with unpaired electrons are paramagnetic (weakly attracted to a magnetic field), while those with all electrons paired are diamagnetic — directly connecting back to the Pauli exclusion principle and Hund’s rule of Chapter 1.
4. Lanthanides and actinides
The lanthanides (filling 4f orbitals) show remarkably similar chemistry to one another because the 4f orbitals are deeply buried and shielded, barely influencing bonding — their most common oxidation state is +3. The actinides (filling 5f orbitals) show more variable oxidation states than the lanthanides because the 5f orbitals are less deeply buried and participate more in bonding; many actinides are radioactive (connecting to Physics Chapter IV), which significantly complicates their study and chemistry.
5. Important compounds and reactions
At syllabus level, familiarity with a few representative transition-metal compounds and reactions (such as common oxidation-state interconversions of iron, manganese, or chromium in redox titrations) illustrates the general principles above in concrete, examinable form, rather than requiring exhaustive coverage of every transition element’s chemistry.
Key Takeaways
- Partially filled d-orbitals, close in energy to the ns subshell, are the root cause of transition metals’ variable oxidation states, complex-forming ability, colour, and magnetism.
- Coordination complexes consist of a central metal ion and surrounding ligands, with coordination number determining geometry.
- Colour arises from ligand-induced d-orbital splitting (crystal field splitting) and light absorption during d–d transitions; magnetism depends on the number of unpaired d-electrons.
- Lanthanides (buried 4f orbitals, mostly +3 state) and actinides (less-buried 5f orbitals, more variable states, often radioactive) both show characteristic within-series similarity, but for different underlying reasons.