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 has its lobes aligned along the x and y axes. • This orbital consists of a dumb–bell surrounded by a small doughnut shaped ring at its waist • This orbital is aligned along the z axis. 1.3 Electronic Configurations of the Elements • The principles that govern the distribution of electrons in the orbitals are: • Recall from Atomic Structure: o Electrons occupy the 4s subshell before the 3d subshell. • Electronic configuration of the first-row d-block elements Element ‘Electrons-in-boxes’ diagram Electronic configuration 3d 4s 21Sc [Ar] 1s2 2s2 2p6 3s2 3p6 3d1 4s2 22Ti [Ar] 1s2 2s2 2p6 3s2 3p6 3d2 4s2 23V [Ar] 1s2 2s2 2p6 3s2 3p6 3d3 4s2 24Cr [Ar] 1s2 2s2 2p6 3s2 3p6 3d5 4s1 25Mn [Ar] 1s2 2s2 2p6 3s2 3p6 3d5 4s2 26Fe [Ar] 1s2 2s2 2p6 3s2 3p6 3d6 4s2 27Co [Ar] 1s2 2s2 2p6 3s2 3p6 3d7 4s2 28Ni [Ar] 1s2 2s2 2p6 3s2 3p6 3d8 4s2 29Cu [Ar] 1s2 2s2 2p6 3s2 3p6 3d10 4s1 30Zn [Ar] 1s2 2s2 2p6 3s2 3p6 3d10 4s2 The Aufbau (building-up) Principle • Electrons fill orbitals from the lowest energy orbital upwards. The Pauli Exclusion Principle • No more than two electrons may occupy a single orbital and, if two electrons do occupy a single orbital, their spins must be opposite. Hund’s Rule • Orbitals of a sub -shell (degenerate) must be occupied singly and with parallel spins before they can be occupied in pairs (with opposite spins). 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 7s

-3- Two exceptions: 1. Electronic configuration of chromium: 1s2 2s2 2p6 3s2 3p6 3d5 4s1 For Cr, the configuration [Ar] 3d5 4s1 is more stable than [Ar] 3d4 4s2. ✓ more stable  less stable 2. Electronic configuration of copper: 1s2 2s2 2p6 3s2 3p6 3d10 4s1 For Cu, the configuration [Ar] 3d10 4s1 is more stable than [Ar] 3d9 4s2. ✓ more stable  less stable 1.4 Electronic Configurations of the Ions • In the formation of cations, the 4s electrons, which are at a higher energy level than the 3d electrons, are removed first. • To write the electronic configuration of ions, Step 1: Write out the electronic configuration of element, filling the 4s orbital first before the 3d orbitals. Step 2: Remove the 4s electrons first before the 3d electrons. Example: Electronic configuration Electronic configuration Cr 1s2 2s2 2p6 3s2 3p6 3d5 4s1 Fe 1s2 2s2 2p6 3s2 3p6 3d6 4s2 Cr+ 1s2 2s2 2p6 3s2 3p6 3d5 Fe+ 1s2 2s2 2p6 3s2 3p6 3d6 4s1 Cr2+ 1s2 2s2 2p6 3s2 3p6 3d4 Fe2+ 1s2 2s2 2p6 3s2 3p6 3d6 Cr3+ 1s2 2s2 2p6 3s2 3p6 3d3 Fe3+ 1s2 2s2 2p6 3s2 3p6 3d5  Exercise 1 For which transition element does its ground state atom have an unpaired electron in its s orbital? A chromium B cobalt C iron D manganese  Exercise 2 Which one of the following represents the electronic configuration of the +1 ion of an element with atomic number 29? A 1s2 2s2 2p6 3s2 3p6 3d9 4s2 C 1s2 2s2 2p6 3s2 3p6 3d10 4s1 B 1s2 2s2 2p6 3s2 3p6 3d9 4s1 D 1s2 2s2 2p6 3s2 3p6 3d10

-4- 1.5 The Transition Elements A transition element is a d-block element which can form one or more stable ions with a partially filled d subshell. • Based on the above definition, which first row d-block elements are not transition elements? Scandium and zinc are not transition elements since they do not form any stable ions with a partially filled d subshell. They ONLY form Sc 3+ and Zn 2+ ions respectively that do not have partially filled d subshell. Sc 1s2 2s2 2p6 3s2 3p6 3d1 4s2 Zn 1s2 2s2 2p6 3s2 3p6 3d10 4s2 Sc3+ 1s2 2s2 2p6 3s2 3p6 Zn2+ 1s2 2s2 2p6 3s2 3p6 3d10 • The element copper forms Cu + ion (electronic configuration: 1s 2 2s2 2p6 3s2 3p6 3d10) which has a fully filled d subshell. Is copper a transition element? Copper is still regarded as a transition element because it can form Cu2+ ion with a partially filled d subshell. Cu 1s2 2s2 2p6 3s2 3p6 3d10 4s1 Cu2+ 1s2 2s2 2p6 3s2 3p6 3d9 2. GENERAL PHYSICAL PROPERTIES • The first-row transition elements are all metals and all elements are remarkably similar in their physical properties. ➢ they are hard and have high densities ➢ they have high melting and boiling points ➢ they are good conductors of heat and electricity 2.1 Atomic Radius The atomic radii of the first -row transition elements (in particular, from V to Cu) in Period 4 are almost constant / relati