RI 2025 Transition Elements

-1- Raffles Institution Year 6 Chemistry 2025 Lecture Notes 22 – An Introduction to the Chemistry of Transition Elements Learning Outcomes Candidates should be able to: (a) explain what is meant by a transition element, in terms of d -block elements forming one or more stable ions with partially filled d subshells (b) state the electronic configuration of a first row transition element and of its ions (c) explain why atomic radii and first ionisation energies of the transition elements are relatively invariant (d) contrast, qualitatively, the melting point and density of the transition elements with those of calcium as a typical s-block element (e) describe the tendency of transition elements to have variable oxidation states (f) predict from a given electronic configuration, the likely oxidation states of a transition element (g) describe and explain the use of Fe3+/Fe2+, MnO4–/Mn2+ and Cr2O72–/Cr3+ as examples of redox systems (h) predict, using E values, the likelihood of redox reactions (see Electrochemistry I) (i) define the terms ligand and complex as exemplified by the complexes of copper (II) ions with water, ammonia and chloride ions as ligands (including the transition metal complexes found in the Qualitative Analysis Notes) (j) explain qualitatively that ligand exchange may occur, as exemplified by the formation of the complexes in (i), including the colour changes involved, and CO/O2 exchange in haemoglobin (k) describe, using the shape and orientation of the d orbitals, the splitting of degenerate d orbitals into two energy levels in octahedral complexes (l) explain in terms of d orbital splitting and d -d transition, why transition element complexes are usually coloured [knowledge of the relative order of ligand field strength is not required] (m) explain how some transition metals and/or their compounds can act as catalysts (see Reaction Kinetics) 1. INTRODUCTION 1.1 The d-block elements • These elements have atoms in which either the inner 3d, 4d, 5d or 6d subshells are being filled up. Their atoms are characterised by ‘inner building’ of d subshells. • We will focus on the first row of the d-block elements. • The 3d row contains ten elements, because the 3d subshell contains five orbitals, each able to accommodate up to two electrons. These elements involve the filling of the 3d orbitals. • They are all metals and tend to be hard metals compared with the s-block metals. 1s 1s 2s 2p 3s 3p 4s 4p 5s 6s 7s 3d 4d 5d 6d 5p 6p 4f 5f Period number 1 2 3 4 5 6 7

-2- 1.2 Shapes of the 3d orbitals • There are five d orbitals (i.e. 𝑑𝑥𝑧, 𝑑𝑥𝑦, 𝑑𝑦𝑧, 𝑑𝑥2−𝑦2 and 𝑑𝑧2) in a d subshell. 𝑑𝑥𝑧 𝑑𝑥𝑦 𝑑𝑦𝑧 𝑑𝑥2−𝑦2 𝑑𝑧2 • These three orbitals have a similar 4–lobed shape. • These orbitals have their lobes pointing between the axes. • This orbital also has a 4–lobed shape but it ha