TUTORIAL CHEMISTRY TEXTS 9 Atomic Structure and Periodicity JACK BARRETT Imperial College of Science, Technology and Medicine, University of London RSC ROYAL SOCIETY OF CHEMISTRY
Cover images Murray Robertsonlvisual elements 1998-99, taken from the 19 Visual Elements Periodic Table, available at www.chemsoc.ot glviselements ISBN -8544-657-7 A catalogue record for this book is available from the British Library The Royal Society of Chemistry 22 Published by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 OWF, UK Registered Charity No. 2789 For further information see our web site at www.rsc.org Typeset in Great Britain by Wyvern 2 I, Bristol Printed and bound by Polestar Wheatons Ltd, Exeter
Preface This book deals with the fundamental basis of the modern periodic classification of the elements and includes a discussion of the periodicities of some atomic properties and the nature of the fluorides and oxides of the elements. An introductory chapter deals with the chemically important fundamental particles, the nature of electromagnetic radiation and the restrictions on our knowledge of atomic particles imposed by Heisenberg s uncertainty principle. Atomic orbitals are described with the minimum of mathematics, and then used to describe the electronic configurations of the elements and the construction of the Periodic Table. A chapter is devoted to the periodicities of the ionization energies, electron attachment energies, sizes and electronegativity coefficients of the elements. There is also a section on relativistic effects on atomic properties. A brief overview of chemical bonding is included as the basis of the remaining chapters, which describe the nature and stoichiometries of the fluorides and oxides of the elements. The book represents an attempt to present the periodicities of properties of the elements in a manner that is understandable from a knowledge of the electronic patterns on which the Periodic Table is based. It should be suitable for an introductory course on the subject and should give the reader a general idea of how the properties of atoms and some of their compounds vary across the periods and down the groups of the classification. This knowledge and understanding is essential for chemists who might very well find exceptions to the general rules described; such events being a great attraction in the continuing development of the subject. Apart from the underlying theoretical content, the general trends in periodicity of the elements may be appreciated by the simple statement that size matters, and so does charge. I thank Ellis Horwood for permission to use some material from my previous books, and Martyn Berry for helpful comments on the manuscript. I am very grateful to Pekka Pyykko for his comments on the section about relativistic effects. Jack Barrett London iii
TUTORIAL CHEMISTRY TEXTS EDITOR-IN-CHIEF ~ Professor E W A he1 EXEC I1 T I V E ED I TO R S ~ ~ Profissor A G Duvies Mr M Bmy Professor D Phillips Profissor J D Woollins EDlJCAT ONAL CONSULTANT This series of books consists of short, single-topic or modular texts, conceni rating on the fundamental areas of chemistry taught in undergraduate science courses. Each book provides a concise account of the basic principles underlying a given subject, embodying an independentlearning philosophy and including worked examples. The one topic, one book approach ensures that the series is adaptable to chemistry cpurses across a variety of institutions. TITLES IN THE SERIES FORTHCOM I NG TITLES Stereochemistry D C Morris Reactions and Characterization of Solids S E Dann Main Group Chemistry W HcJnderson d- and f-block Chemistry C J Joncs Structure and Bonding J Burrett Functional Group Chemistry J R Hunson Organotransition Metal Chemistry A F Hill Heterocyclic Chemistry M Sainshury Atomic Structure and Periodicity J Burrett Thermodynamics and Statistical Mechanics J M Setldon and J D Gule Basic Atomic and Moleciilar Spectroscopy Aromatic Chemistry Organic Synthetic Methods Quantum Mechanics for Chemists Mechanisms in Organic Reactions Molecular Interactions Reaction Kinetics X-ray Crystallography Lanthanide and Actinide Elements Maths for Chemists Bioinorganic Chemistry Chemistry of Solid Surfa8:es Biology for Chemists Mu1 ti-elemen t N MR Further infortnution about this series is availuble ut NWW. chenisoc. orgltct Orders und enquiries should be sent to: Sales and Customer Care, Royal Society of Chemistry, Thomas Graham Hcuse, Science Park, Milton Road, Cambridge CB4 OWF, UK Tel: +44 I 223 43236; Fax: +44 122342617; Email: sales@rsc.org
Contents I Atomic Particles, Photons and the Quantization of Electron Energies; Heisenberg s Uncertainty Principle 1,l Fundamental Particles 1 1.2 Electromagnetic Radiation 5 1.3 The Photoelectric Effect 8 1.4 Wave-Particle Duality 11 1.5 The Bohr Frequency Condition 13 1.6 The Hydrogen Atom 14 1.7 The Observation of Electrons; the Heisenberg Uncertainty Principle 17 2 Atomic Orbitals 21 2.1 The Hydrogen Atom 21 3 The Electronic Configurations of Atoms; the Periodic Classification of the Elements 35 3.1 Polyelectronic Atoms 36 3.2 The Electronic Configurations and Periodic Classification of the Elements 42 3.3 The Electronic Configurations of Elements Beyond Neon 5 3.4 The Periodic Table Summarized 56 I V
vi Contents 4 Periodicity I: Some Atomic Properties; Relativistic Effects 59 4.1 4.2 4.3 4.4 4.5 Periodicity of Ionization Energies Variations in Electron Attachment Energies Variations in Atomic Size Electronegativi ty Relativistic Effects 59 7 74 85 91 5 Periodicity II: Valencies and Oxidation States 99 5.1 5.2 5.3 5.4 5.5 5.6 An Overview of Chemical Bonding Valency and Oxidation State: Differences of Terminology Valency and the Octet and 18-Electron Rules Periodicity of Valency and Oxidation States in the s- and p-block Elements Oxidation States of the Transition Elements Oxidation States of the f-block Elements 99 19 111 117 12 123 6 Periodicity 111: Standard Enthalpies of Atomization of the Elements 127 7 6.1 7.1 7.2 7.3 Periodicity of the Standard Enthalpies of Atomization of the Elements Periodicity I V Fluorides and Oxides The Fluorides and Oxides of the Elements Fluorides of the Elements Oxides of the Elements Answers to Problems Subject Index 127 142 142 143 152 168 177
Atomic Particles, Photons and the Quantization of Electron Energies; Heisenberg s Uncertainty Principle As an introduction to the main topics of this book - atomic structure and the periodicity of atomic properties - the foundations of the subject, which lie in quantum mechanics, and the nature of atomic particles and electromagnetic radiation are described. By the end of this chapter you should understand: Which fundamental particles are important in chemistry The nature of electromagnetic radiation The photoelectric effect Wave-particle duality The relationship of electromagnetic radiation to changes of energy in nuclei, atoms, molecules and metals: the Bohr frequency condition The main features of the emission spectrum of the hydrogen atom and the quantization of the energies permitted for electrons in atoms Heisenberg s Uncertainty Principle and the necessity for quantum mechanics in the study of atomic structure l m i Fundamental Particles To describe adequately the chemical properties of atoms and molecules it is necessary only to consider three fundamental particles: protons and neutrons, which are contained by atomic nuclei, and electrons which surround the nuclei. Protons and neutrons are composite particles, each consisting of three quarks, and are therefore not fundamental particles in the true sense of that term. They may, however, be regarded as being 1
~~~ 2 Atomic Structure and Periodicity The positive electron or positron has a mass identical to the has an opposite charge. Positrons are emitted by some radioactive nuclei and perish when they meet a negative electron, the two particles disappearing completely to form two y-ray photons. Such radiation is known as annihilation radiation: 2e- -+ 2hv fundamental particles for all chemical purposes. The physical properties of electrons, protons and neutrons are given in Table 1.1, together with Table 1.1 Properties of some fundamental particles and the hydrogen atom (e is the elementary unit of electronic charge = 1.621 7733 x 1-19 coulombs) Particle Symbol M~SS~/?@~~ kg RAM Charge Spin Proton P 16726.231 1.72765 +e tz Neutron n 16749.286 1.86649 zero t? Electron e 9.193897.5486 -e t? H atom H 16735.339 1.7825 zero - a The atomic unit of mass is given by mu = m(12c)/12 kg. The relative atomic mass (RAM) of an element is given by melement/mu, where melement is the mass of one atom of the element in kg. The absolute masses are given, together with their values on the relative atomic mass (RAM) scale, which is based on the unit of mass being equal to that of one twelfth of the mass of the I2C isotope, i.e. the RAM of 12C = 12. exactly. The spin values of the particles are important in determining the behaviour of nuclei in compounds when subjected to nuclear magnetic resonance spectroscopy (NMR) and electron spin resonance spectroscopy (ESR). The conventional way of representing an atom of an element and its nuclear properties is by placing the mass number, A [the whole number closest to the accurate relative mass (equal to the sum of the numbers of protons and neutrons in the nucleus)], as a lefthand superscript to the element symbol, with the nuclear charge, Ze+, expressed as the number of protons in the nucleus (the atomic number, 2) placed as a left-hand subscript, $X, where X is the chemical symbol for the element. The value of A - 2 is the number of neutrons in the nucleus. Mass numbers are very close to being whole numbers because the relative masses of nuclei are composed of numbers of protons and neutrons whose relative masses are very close to 1 on the RAM scale (see Table 1.1). The actual mass of an atom, M, can be expressed by the equation: