NJC 4 Physical Periodicity

National Junior College SH1 H1 Chemistry 1 PHYSICAL PERIODICITY OF ELEMENTS Content: Periodicity of atomic properties of the elements: variation with proton number across the third period (sodium to chlorine) and down the group of: • atomic radius and ionic radius • ionisation energy • electronegativity Learning Outcomes required for H1 (8873) Chemistry: Trends and variations in atomic properties For elements in the third period (sodium to chlorine), and in Group 17 (chlorine to iodine) candidates should be able to: (l) Recognize variation in the electronic configurations across a Period and down a Group. (m) describe and explain qualitatively the trends and variations in atomic radius, ionic radius, first ionisation energy and electronegativity: (i) across a Period in terms of shielding and nuclear charge (ii) down a Group in terms of increasing number of filled electronic shells, shielding and nuclear charge References 1. Hill, G. & Holman, J. (1992). Chemistry in Context, 6th edition. New York: Prentice Hall. 2. Ramsden, E. N. (1994). A Level Chemistry, 3rd edition. UK: Stanley Thornes (Publishers) Ltd. 3. Chemistry for Advanced Level, Peter Cann, Peter Hughes 4. A-level Chemistry, E.N. Ramsden 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. Copyright © 2022 National Junior College

National Junior College SH1 H1 Chemistry 2 1 INTRODUCTION TO THE PERIODIC TABLE Mankind had always been seeking to discover new chemical elements since centuries ago. As of January 2021 , a total o f 118 elements have been identified. Studying these hundreds of elements with different physical and chemical properties will surely be a daunting task, unless there is a systematic and organized way to do it. Much of what we will cover in this topic is credited to the Russian chemist Dmitr i Ivanovich Mendeleev (1834 -1907),whose seminal hypothesis that the chemical and physical properties of the elements vary in a periodic way laid the foundations of the modern Periodic Table. The Periodic Table is a table of elements arranged by order of their atomic numbers in such a way that the periodic properties (chemical periodicity) of the elements are made clear. In fact, the Periodic Table is a masterpiece of organised chemical information. The standard form of Periodic Table includes periods (horizontal rows) and groups (vertical columns). The elements in the same period have the same number of core electrons as that of the noble gas from previous period. Elements in the same group have the same number of valence electrons and hence similar chemical properties. Physical periodicity is the study of periodic trends which indicate the tendencies of certain elemental characteristics to change as one progresses along a row or column of the Periodic Table.

National Junior College SH1 H1 Chemistry 3 The valence (outermost) shell electronic configuration of the elements can b e used to determine the location of the element in the Periodic Table. • The principal quantum number, “n”, of the valence shell indicates the period of the element. • The number of valence electrons indicates the group of the element. (Note that elements with 3-8 valence electrons are in Group 13 to 18) Group Valence shell electronic configuration 1 ns1 2 ns2 13 ns2 np1 14 ns2 np2 15 ns2 np3 16 ns2 np4 17 ns2 np5 18 ns2 np6 For example, an element with the electronic configuration of 1s22s22p63s23p63d104s24p5 is located at Period 4 of the Periodic Table and is in Group 17. The element is Br. Checkpoint 1 Write the electronic configurations of (i) Li and Na and (ii) F and Cl. Observe their relative positions on the Periodic Table. Underline the valence electronic configurations. (i) Li : ____________ Na: _______________ (ii) F : ____________ Cl : ____________________

National Junior College SH1 H1 Chemistry 4 Note: Electrons in the same outer electronic shell exert negligible shielding effect. 2 FUNDAMENTAL CONSIDERATIONS IN IONISATION ENERGY AND PHYSICAL PERIODICITY 2.1 Nuclear charge Nuclear charge is the total charge of all the protons in the nucleus. The larger the number of protons in the nucleus of an atom, the greater the nuclear charge. Hence, across a period or down a group, nuclear charge increases. 2.2 Shielding effect Valence electrons in an atom do not only ‘feel’ attraction from the protons in the nucleus, but also repulsion from inner shells electrons. The ‘feel’ of repulsion from i nner-shell electrons is called shielding effect. The inner-shell electrons shield the outer electron from the full charge of the nucleus. Shielding effect increases as the number of inner shell electrons increases. 2.3 Distance of electrons from nucleus The larger the number of filled electronic shells an atom has, the further the valence electrons are from the nucleus, hence the weaker the attraction they experience from the proton in the nucleus. 2.4 Nuclear attraction (“net attraction electrons experienced from the nucleus”) Nuclear attraction refers to “attraction between the nucleus and the electrons (nuclear charge) – “repulsion between electrons (shielding effect) and distance of electrons from nucleus” Nuclear attraction = nuclear charge − shielding effect and distance of electrons from nucleus

National Junior College SH1 H1 Chemistry 5 3 ATOMIC RADIUS (ATOMIC SIZE) The atomic radius of an element is a measure of the size of its atoms, usually the mean distance from the nucleus to the boundary of space occupied by the valence electrons. Since the boundary is not a w ell- defined physical entity, there are various non -equivalent definitions of atomic radius. Some commonly used definitions of atomic radius include metallic radius, covalent radius and van der Waals radius. The metallic radius is defined as half the internuclei distance between the two adjacent metal atoms in the metallic lattice. The covalent radius is defined as half the internuclei distance between two atoms which are covalently bonded. In principle, the sum of the two covalent radii should equal the covalent bond length between two atoms. For noble gas such as He, Ne and Ar, the measured radius is known as the van der Waals radius as these noble gas atoms do not form covalent bond. Atomic and ionic radii from the Data Booklet 0.380 nm 0.372 nm 0.198 nm Metallic radius of Na = 0.186 nm Covalent radius of Cl = 0.099 nm van der Waals radius of Ar = 0.190 nm

National Junior College SH1 H1 Chemistry 6 In this section we refer atomic radius to the size of its atoms in general. Atomic Size Trends 4 PERIODICITY TREND 4.1 Across the period - Atomic radius and Ionic radius - Ionisation Energy - Electronegativity 4.2 Down the group - Atomic radius and Ionic radius - Ionisation Energy - Electronegativity 4.3 Sharp drop in first IE between Group 18 element and Group 1 element of the next period Learning Outcome: Describe and explain qualitatively the trends and variations in atomic radius, ionic radius, first ionisation energy and electronegativity: (i) across a Period in terms of shielding effect and nuclear charge (ii) down a Group in terms of increasing number of electronic shells , shielding effect and nuclear charge

National Junior College SH1 H1 Chemistry 7 Period 3 Sodium (Na) Magnesium (Mg) Alumin