The d- and f-Block ElementsMind Map

Transition elements are elements whose atoms or common ions contain partially filled d subshells. They occupy the central region of the periodic table, mainly groups 3 to 12. Their name reflects their position between highly electropositive s-block metals and more electronegative p-block elements. They show metallic character, high melting points, high densities, variable oxidation states, coloured ions, paramagnetism, complex formation, catalytic activity and alloy formation. These properties arise because ns and (n-1)d electrons have comparable energies and can participate in bonding. NEET frequently asks why Zn, Cd and Hg are not typical transition elements, why transition metals act as catalysts, and why they show variable oxidation states.

Electronic configuration is the foundation of d- and f-block chemistry. In d-block elements, electrons enter the penultimate (n-1)d subshell while ns electrons are also present. In f-block elements, electrons enter inner (n-2)f orbitals. Because ns and (n-1)d orbitals have close energies, d-block elements show variable oxidation states. Cr and Cu are key exceptions: chromium is [Ar] 3d5 4s1 and copper is [Ar] 3d10 4s1 due to extra stability of half-filled and fully-filled d subshells. Lanthanoids generally fill 4f orbitals and commonly form Ln3+, while actinoids fill 5f orbitals and show more variable oxidation states.

The general properties of d-block elements arise from partially filled d orbitals and strong metallic bonding. Atomic and ionic radii decrease initially across a series due to increasing nuclear charge, then become nearly constant because d-electron shielding offsets the attraction. Ionisation enthalpies generally increase across the series but show irregularities due to electronic configuration stability. Many transition metal ions are coloured because electrons absorb visible light and undergo d-d transitions. Magnetic behaviour depends on unpaired electrons; more unpaired electrons mean stronger paramagnetism. They form complexes due to small size, high charge density and vacant orbitals. They also form interstitial compounds and alloys due to suitable crystal structures and similar atomic sizes.

The most important compounds in this chapter are potassium dichromate, K2Cr2O7, and potassium permanganate, KMnO4. Both are strong oxidising agents and are frequently tested in NEET through preparation, colour, structure-related facts, reactions and medium-dependent redox products. Potassium dichromate is prepared from chromite ore through sodium chromate and sodium dichromate, followed by conversion to potassium dichromate. It is orange and interconverts with yellow chromate depending on pH. Potassium permanganate is prepared from pyrolusite ore through potassium manganate and then oxidation to permanganate. KMnO4 is purple and acts differently in acidic, neutral and alkaline media, giving Mn2+, MnO2 or MnO4^2- respectively.

Lanthanoids and actinoids are f-block or inner transition elements. Lanthanoids involve progressive filling of 4f orbitals and generally show +3 oxidation state. They are silvery metals, electropositive and show gradual decrease in Ln3+ ionic radii from La3+ to Lu3+, called lanthanoid contraction. This happens because 4f electrons shield nuclear charge poorly. Actinoids involve filling of 5f orbitals, are generally radioactive, and show wider oxidation states because 5f, 6d and 7s orbitals have comparable energies. NEET frequently asks lanthanoid contraction, its consequences, +2/+4 exceptions among lanthanoids, comparison of lanthanoids and actinoids, and applications such as misch metal and nuclear fuels.