ASRJC 2025 H2 Chem Transition Elements Notes

©2025ASRJC/CHEM 1 ANDERSON SERANGOON JUNIOR COLLEGE 2025 JC2 H2 Chemistry AN INTRODUCTION TO THE CHEMISTRY OF TRANSITION ELEMENTS 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 Fe3+/Fe2+, MnO 4–/Mn2+ and Cr 2O72–/Cr3+ as examples of redox systems (h) predict, using E 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 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 (see also 8(j)) References 1. Chemistry for Advanced Level, Peter Cann and Peter Hughes 2. Understanding Advanced Physical Inorganic Chemistry, The Learner’s Approach, Jeanne Tan and Chan Kim Seng.

©2025ASRJC/CHEM 2 1. INTRODUCTION In the Periodic Table, the d block elements are found in groups 3 to 12. A transition element is a d block element that can form one or more stable ions with _______________ d subshells. The ten elements from scandium (Sc) to zinc (Zn) comprise the 3d block of elements. Based on the definition in our syllabus, we will focus on the following eight elements. Ti V Cr Mn Fe Co Ni Cu titanium vanadium chromium manganese iron cobalt nickel copper Sc and Zn are not considered transition elements as they do not form stable ions with partially filled d subshells. Sc [Ar] 3d1 4s2 Zn [Ar] 3d10 4s2 Sc3+ [Ar] Zn2+ [Ar] 3d10 The common stable ion for Sc is Sc3+, which has an empty d subshell. As for Zn, its only stable ion Zn2+, has a completely filled d subshell. It is the presence of the partially filled d subshell that gives rise to the distinct properties of transition elements. Typical chemical properties of transition elements include the ability to form coloured ions, exist in variable oxidation states, form complexes and act as catalysts.

©2025ASRJC/CHEM 3 1.1 Electronic Configuration Table 1 below shows the electronic configurations of the first row d block elements. [Ar] represents electronic configuration (1s2 2s2 2p6 3s2 3p6) * anomalous electronic configurations Table 1: Electronic configurations of the first row d block elements ATOMIC STRUCTURE  The 4s orbital is filled with electrons first before the 3d orbitals. This is because the empty 4s orbital has a lower energy than the 3d orbitals.  Exceptions in Cr and Cu : Electronic configurations with half –filled or completely filled d orbitals are unusually stable as both these configurations result in a symmetrical charge distribution around the nucleus. Hence, the electronic configuration of Cr is [Ar]3d 54s1 (instead of [Ar]3d44s2) and Cu is [Ar]3d104s1 (instead of [Ar]3d94s2).  When the 3d subshell is occupied by electrons, the 3d electrons repel the 4s electrons further away from the nucleus and cause the 4s electrons to be at a higher energy level. So when transition elements form ions, electrons are first removed from the 4s subshell before removing from the 3d subshell.  Hence, w hen writing the electronic configuration of the cation s, write the electronic configuration of the neutral atom first and then remove the electrons from the 4s subshell first before the 3d subshell. Symbol Atomic (proton) Number Electronic Configuration 3d 4s Sc 21 [Ar] 3d1 4s2 [Ar] Ti 22 [Ar] 3d2 4s2 [Ar] V 23 [Ar] 3d3 4s2 [Ar] Cr* 24 [Ar] ______ [Ar] Mn 25 [Ar] 3d5 4s2 [Ar] Fe 26 [Ar] 3d6 4s2 [Ar] Co 27 [Ar] 3d7 4s2 [Ar] Ni 28 [Ar] 3d8 4s2 [Ar] Cu* 29 [Ar] ______ [Ar] Zn 30 [Ar] 3d10 4s2 [Ar]

©2025ASRJC/CHEM 4 Exercise 1 Write the electronic configuration of the following ions. 24Cr 1s2 2s2 2p6 3s2 3p6 3d5 4s1 24Cr2+ 1s2 2s2 2p6 3s2 3p6 3d4 26Fe 26Fe2+ 29Cu 29Cu+ 2. PHYSICAL PROPERTIES 2.1 Atomic radius and First Ionisation Energy Atomic radii and first ionisation energies of transition elements are relatively invariant. Figure 1: Trend of atomic radius Figure 2: Trend of ionisation energies Reason: • The nuclear charge increases from Ti to Cu as the number of protons increases. • However, additional electrons are being added to the inner 3d orbitals which causes an increase in shielding effect. • As a result, the increase in shielding effect nullifies / cancels out the effect due to the increase in nuclear charge. • Hence, the electrostatic attraction of the valence electrons from the nucleus increases only slightly or remains fairly constant. • So, the atomic radii across the transition elements decreases very slightly or remains fairly constant. • So, similar amount of energy is needed to remove the 4s valence electron from the elements, and • the first ionisation energies across the transition elements increases very slightly or remains fairly constant.

©2025ASRJC/CHEM 5 2.2 Melting point and density Figure 3: Melting points of Ca and the d block elements Figure 4: Densities of Ca and the d block elements Transition elements have higher melting points compared to s block elements, e.g. Ca  Both s block elements and transition elements consist of a giant metallic lattice structure held together by strong metallic bonds.  In transition elements, both the 3d and 4s electrons are available for metallic bonds since the energy level difference between the 3d and 4s orbitals is small.  In s block elements, only one or two 4s electrons is/are available for metallic bondi ng. (e.g. calcium metal contributes its two 4s electrons)  The greater number of delocalised electrons available for metallic bonding in transition elements results in stronger electrostatic attraction between the positively charged ions and the ‘sea’ of delocalised electrons.  More energy is required to overcome the stronger metallic bonds in transition elements as compared to the s block elements.  Exception: Manganese has a stable d5 arrangement ([Ar]3d54s2) which results in the low availability of electrons for delocalisation and hence a relatively low melting point. Transition elements have higher densities compared to s block elements, e.g. Ca  Transition elements have relatively smaller atomic radii and higher atomic mass compared to s block elements.  Also, transition elements have more closely packed structures due to their stronger metallic bonding as compared to s block elements.  Thus, transition elements are denser than s block elements. For self-reading