NYJC H2 Chem Transition Elements Lecture Notes

1 Nanyang Junior College H2 Chemistry (9729) Lecture Notes 20 An Introduction to the Chemistry of Transition Elements Lecturer: Ms Theresia Line Ishak JC2/2025 Content • General physical and characteristic chemical properties of the first set of transition elements, titanium to copper • Colour of complexes 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 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 Fe 3+/Fe2+, MnO4–/Mn2+ and Cr2O72–/Cr3+ as examples of redox systems (h) predict, using Eo values, the likelihood of redox reactions (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 elements and/or their compounds can act as catalysts References Peter Cann and Peter Hughes. (2002). Chemistry for Advanced Level (pp. 563-581). John Murray (Publishers) Ltd

2 1. INTRODUCTION Our study focuses on the first row of the transition elements (in Period 4). 1.1 Electronic Configuration • Apply Hund’s Rule, Aufbau’s Rule and Pauli’s Exclusion Principle when filling electrons into the orbitals. (Refer to Atomic Structure Lecture Notes) • The 4s subshell is filled with electrons first before the 3d subshell. Reason: The empty 4s subshell has a lower energy than the 3d subshell. Example: 21Sc: 1s22s22p63s23p63d14s2 22Ti: 1s22s22p63s23p63d24s2 23V: 1s22s22p63s23p63d34s2 • Exception: Cr and Cu 24Cr: [Ar]3d54s1 NOT [Ar]3d44s2 29Cu: [Ar]3d104s1 NOT [Ar]3d94s2 Reason: An exactly half-filled or completely filled d subshell is more favoured than a partially filled d subshell. When exactly half-filled or fully filled, the distribution of electron density is more symmetrical around the nucleus. This is more stable compared to the partially filled d subshell. Note that even though we fill 4s subshell with electrons before 3d subshell, we write 3d first, then 4s. d block elements

3 • Electronic configuration of d-block elements 3d 4s • Electronic configuration of d-block cations When we write the electronic configuration of d -block cation s, we first write the electronic configuration of the neutral atom, then we remove electrons from the 4s subshell, followed by the 3d subshell. Example: 22Ti: 1s22s22p63s23p63d24s2 22Ti2+: 1s22s22p63s23p63d2 23V: 1s22s22p63s23p63d34s2 23V2+: 1s22s22p63s23p63d3 24Cr: [Ar]3d54s1 24Cr3+: [Ar]3d3 29Cu: [Ar]3d104s1 29Cu2+: [Ar]3d9 Example 1 (a) Complete the following to show the ground state electronic configuration of the following ions. (i) Ti3+: 1s22s22p63s23p6_____ (ii) Ni2+: 1s22s22p63s23p6_____ (iii) Cr3+: 1s22s22p63s23p6_____ (b) What is the charge of the following ions? (i) a cobalt ion with d6 configuration: _____ (ii) an iron ion with d6 configuration: _____ (c) Which of the following species does not have either a half-filled or a fully filled 3d subshell? A Zn B Mn2+ C Fe3+ D Co 21Sc 1s22s22p63s23p6 ↑ ↑↓ 22Ti 1s22s22p63s23p6 ↑ ↑ ↑↓ 23V 1s22s22p63s23p6 ↑ ↑ ↑ ↑↓ 24Cr 1s22s22p63s23p6 ↑ ↑ ↑ ↑ ↑ ↑ 25Mn 1s22s22p63s23p6 ↑ ↑ ↑ ↑ ↑ ↑↓ 26Fe 1s22s22p63s23p6 ↑↓ ↑ ↑ ↑ ↑ ↑↓ 27Co 1s22s22p63s23p6 ↑↓ ↑↓ ↑ ↑ ↑ ↑↓ 28Ni 1s22s22p63s23p6 ↑↓ ↑↓ ↑↓ ↑ ↑ ↑↓ 29Cu 1s22s22p63s23p6 ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑ 30Zn 1s22s22p63s23p6 ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ 24Cr: 1s22s22p63s23p63d54s1 NOT 1s22s22p63s23p63d44s2 29Cu: 1s22s22p63s23p63d104s1 NOT 1s22s22p63s23p63d94s2 3d1 3d8 3d3 3+ 2+

4 1.2 Definition of Transition Element • Example: Fe is a transition element. Fe can form Fe2+ and Fe3+, which are stable ions with a partially filled d-subshell. 26Fe2+: 1s22s22p63s23p63d6 26Fe3+: 1s22s22p63s23p63d5 • Sc and Zn are NOT transition elements (even though they are in the d -block). Compounds of Sc and Zn do not show the characteristic properties of transition metals. Reason: Sc forms only one stable ion, Sc3+ that has no electrons in the 3d subshell. 21Sc: [Ar]3d14s2 21Sc3+: [Ar] ► empty 3d subshell Zn forms only one stable ion, Zn2+ that has a fully filled 3d subshell. 30Zn: [Ar]3d104s2 30Zn2+: [Ar]3d10 ► fully filled 3d subshell • Hence, the first transition metal series consists of Ti to Cu in the first row of the d-block. Example 2 Element X has 29 electrons and forms ions with 1+ and 2+ charge. Write the electronic configuration of X+ and X2+ and explain if element X is a transition element. X+ : ____________________________ X2+: ____________________________ Answer: Element X is a transition element as X can form 2 stable ions and X2+ has a partially filled 3d subshell. A transition element is a d-block element that can form one or more stable ions with a partially filled d-subshell. 1s22s22p63s23p63d10 1s22s22p63s23p63d9 Note that element X is Cu. 29Cu: [Ar]3d104s1

5 2. PHYSICAL PROPERTIES Across the d -block, the elements have similar physical properties, but they show significant differences from s -block elements such as Ca. The d -block elements are harder, have higher densities and have higher melting points. 2.1 Comparison across Transition Elements (a) First Ionisation Energies M(g) → M+(g) + e– Across Transition Elements Across Period 3 Elements Trend Relatively invariant Generally increase Explanation • From Sc to Cu, as number of protons increases, nuclear charge increases. • Each additional electron is added to the penultimate 3d subshell. Shielding effect increases as the presence of 3d orbitals shield the 4s electrons from the nuclear attraction. • Hence, effective nuclear charge remains almost constant ( only increases very slightly). • Attraction between the nucleus and the outermost electron remains almost constant. • Energy required to remove the outermost 4s electron is relatively invariant. • As number of protons increases, nuclear charge increases. • As successive electrons are added to the same outermost shell, shielding effect remains approximately the same. • Hence, effective nuclear charge increases and attraction between the nucleus and outermost electron increases. • More energy is required to remove the outermost electron. • Exceptions: - ns2 vs ns2 np1 configuration - ns2 np3 vs ns2 np4 configuration (b) Atomic Radii Across Transition Elements Across P