NJC 2026 Transition Elements Notes Student

National Junior College SH2 H2 Chemistry 1 AN INTRODUCTION TO THE CHEMISTRY OF TRANSITION ELEMENTS Content • General physical and characteristic chemical properties of the first set of transition elements, scandium to copper • Colour of complexes Learning Outcomes Candidates should be able to: (a) explain that a transition element is a d block element whose atom has an incomplete d subshell, or which can give rise to cations with an incomplete d subshell (b) state the electronic configuration of a first row transition element and of its ions (c) explain why the atomic radii and first ionisation energies of the transition metal atoms 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+, MnO 4−/Mn2+ and Cr 2O72−/Cr3+ as examples of redox systems (see also REDOX AND ELECTROCHEMISTRY) (h) predict, using Eꝋ values, the likelihood of redox reactions (see also ELECTROCHEMISTRY) (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 (see also REACTION KINETICS) Copyright © 2026 National Junior College All Rights Reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording or any other information storage and retrieval system, without prior permission in writing from the copyright owner.

National Junior College SH2 H2 Chemistry 2 Note: Ground state electronic configurations of Cr and Cu atom are [Ar] 3d5 4s1 and [Ar] 3d10 4s1 respectively 1 Introduction Success Criteria: • I know the definition of a transition element. • I can state the electronic configuration of a first row transition element and of its ions. A transition element is d block element whose atom has an incomplete d subshell, or which can give rise to cations with an incomplete d subshell. For A−Level syllabus, we study the chemistry of first row transition elements. (e.g. from scandium to copper) H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Rf Db Sg Bh Hs Mt Ds Rg Cn The electronic configurations of d-block elements (first row) are: Element Atomic no. Electronic configuration Electronic configuration of 1 stable ion Scandium 21 [Ar] 3d1 4s2 Sc3+ [Ar] 3d0 4s0 Titanium 22 [Ar] 3d2 4s2 Ti2+ [Ar] 3d2 4s0 Vanadium 23 [Ar] 3d3 4s2 V3+ [Ar] 3d2 4s0 Chromium* 24 [Ar] 3d5 4s1 Cr3+ [Ar] 3d3 4s0 Manganese 25 [Ar] 3d5 4s2 Mn2+ [Ar] 3d5 4s0 Iron 26 [Ar] 3d6 4s2 Fe2+ [Ar] 3d6 4s0 Cobalt 27 [Ar] 3d7 4s2 Co3+ [Ar] 3d6 4s0 Nickel 28 [Ar] 3d8 4s2 Ni2+ [Ar] 3d8 4s0 Copper* 29 [Ar] 3d10 4s1 Cu2+ [Ar] 3d9 4s0 Zinc 30 [Ar] 3d10 4s2 Zn2+ [Ar] 3d10 4s0 Quick recall: • For d−block elements, electrons occupy the 4s subshell first before the 3d subshell as the empty 4s subshell is of lower energy than the empty 3d subshell. • When filled with electrons, the 4s subshell has a higher energy than the 3d subshell. Hence, during the formation of positive ions, the 4s electrons are removed first. d-block elements

National Junior College SH2 H2 Chemistry 3 2 Physical Properties of Transition Elements Success Criteria: • I can explain why the atomic radii and first ionization energies of transition elements of the same period remain relatively constant. • I can explain why the melting points and densities of transition elements are higher than typical s block elements. 2.1 Atomic radii & First Ionisation Energy Across Period 3 Elements (Na to Cl) Across Transition Elements (Sc to Cu) Atomic radii Decrease significantly Remain relatively constant 1st I.E. Increase significantly Remain relatively constant Reasons • Nuclear charge increases • Electrons are added to same valence shell • Shielding effect remains approximately constant • Nuclear attraction for the valence electrons increases significantly • Nuclear charge increases • Electrons are added to the inner principal quantum shell (3d subshell) • Shielding effect for valence shell 4s electrons increases due to the increase in no. of inner shell (3d) electrons • Nuclear attraction for the valence electrons is almost constant Q: Why is zinc not considered as transition elements even though it is a d- block element? • Zn atom has complete d-subshell: Zn: [Ar] 3d10 4s2 • It has only one known oxidation state: Zn2+ , with an incomplete d subshell. Zn2+: [Ar] 3d10 • As such, the melting point of Zn (420 C) is much lower than the others (> 1500 C). 0 0.05 0.1 0.15 0.2 0.25 Na Mg Al Si P S Cl K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Element Atomic radius/nm

National Junior College SH2 H2 Chemistry 4 2.2 Melting Point and Density Differences between an s -block metal, Ca, and a d -block transition element, Fe Calcium [Ar] 4s2 Iron [Ar] 3d6 4s2 Melting point / C 850 1535 Density / g cm−3 1.54 7.86 2.2.1 Melting Point All transition elements have much higher melting and boiling points than the s-block metals due to stronger metallic bonds. The small energy difference between the 3d and 4s subshells allows transition elements to contribute 3d AND 4s electrons to the sea of delocalised electrons for metallic bonding. Whereas calcium only contributes its 4s electrons for metallic bonding. The strength of the metallic bond is proportional to the number of delocalised electrons. More energy is required to overcome the stronger metallic bonding in transition elements than calcium. Transition elements have higher melting point than calcium. 2.2.2 Density Transition elements are denser than the s-block elements in the same period. Transition elements have smaller atomic size (atomic radii in Data Booklet) and thus more atoms per unit volume. Furthermore, the transition element atoms are of high atomic mass compared to s-block elements. Hence transition elements have greater mass per unit volume (higher density) as compared to s-block elements. Checkpoint 1 Which element is most likely to be a transition element? m.p./oC b.p./oC density/g cm−3 A 1900 3400 6.1 B 157 2000 7.3 C 1280 2970 1.9 D 3730 4200 2.2 Transition element has high m.pt and high density.

National Junior College SH2 H2 Chemistry 5 Note: In general, Highest possible O.N. = no of unpaired d-electrons + 4s electrons 3 Chemical Pro