Preview Chemistry of the NonMetals Syntheses Structures Bonding Applications (De Gruyter Textbook) (De Gruyter STEM) by Ralf Steudel (2020)

Preview Chemistry of the NonMetals Syntheses Structures Bonding Applications (De Gruyter Textbook) (De Gruyter STEM) by Ralf Steudel (2020) Preview Chemistry of the NonMetals Syntheses Structures Bonding Applications (De Gruyter Textbook) (De Gruyter STEM) by Ralf Steudel (2020) Preview Chemistry of the NonMetals Syntheses Structures Bonding Applications (De Gruyter Textbook) (De Gruyter STEM) by Ralf Steudel (2020)

Bioinorganic Chemistry.

Rabinovich,

ISBN----, e-ISBN ----

Inorganic Trace Analytics

Trace Element Analysis and Speciation

Matusiewicz, Bulska (Eds.),

Reviews in Inorganic Chemistry

Editor-in-Chief: Schulz, Axel

ISSN-, e-ISSN -

of the Non-Metals

In cooperation with David Scheschkewitz

Library of Congress Control Number: 2019947567

Bibliographic information published by the Deutsche Nationalbibliothek

The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.dnb.de.

© 2020 Walter de Gruyter GmbH, Berlin/Boston

Cover image: Philiphotographer / E+ / getty images

Typesetting: Integra Software Services Pvt Ltd.

Printing and binding: CPI books GmbH, Leck

www.degruyter.com

How would our planet Earth look like without nonmetals? No water, no air, no life!Not even rocks and sand since oxygen and silicon are responsible for 72.7% of themass of crustal rocks on the Earth The human body consists of 96.6% of oxygen,carbon, hydrogen and nitrogen If all the other nonmetallic elements are added, weend up with 98.0% Nonmetal compounds play a crucial role in our daily life and inindustry as you will see in this book It is therefore obvious that the chemistry ofthe nonmetals is a major part of the chemical education at all levels, from highschool to university.

The book you are holding in your hands is very special to me I wrote the firstGerman edition at the very early stage of my academic career, shortly ahead of

my one-year sabbatical leave at M.I.T., Cambridge (Massachusetts) It originatedfrom my lectures at the Technical University Berlin, the place known as a hotspot

of nonmetal chemistry research With the years, the book became my lifetimeproject I was fortunate enough to share my passion and interest in the chemistry

of the nonmetals (and the yellow element in particular) with many friends andcolleagues in academia and industry all over the world Thus, the book was al-ways nourished with the modern developments in fundamental and industrialresearch

Five editions of this monograph have been published by de Gruyter, Berlin/Boston, each time completely modernized and extended This newest interna-tional edition is an updated translation of the latest German edition of 2013 I amgrateful to my two younger colleagues who share the same passion– Prof DavidScheschkewitz at Saarland University (Germany) who contributed in translatingand updating half of the present edition; and Prof Ingo Krossing at the University

of Freiburg (Germany) whose know-how enriched several chapters of a previousGerman edition

The book presents an infinite variety and marvelous chemical subtlety of the

22 elements occupying the upper-right section of the Periodic Table and of gen, which was at the origin of the universe when it started 13.8 billion years agoand from which all other elements were formed by nuclear reactions The work isorganized in two parts: Part I explains the basic theoretical concepts needed tounderstand the structures and reactions of (nonmetallic) molecules and crystals.The larger Part II presents the syntheses, structures and applications of the corre-sponding compounds and materials We also address their significance in dailylife, chemical industry, environment, material science and farming whereverpossible

hydro-Numerous review articles and original publications are cited in footnotes andencourage the readers to study certain topics more extensively To keep the size ofthe footnotes with nearly 1000 references under control, however, only one author

is given if there are more than three In addition, well-established handbooks of

inorganic chemistry (e.g., GMELIN1) and chemical technology (e.g., ULLMANN2

WINNACKER-KÜCHLER3BÜCHEL-MORETTO-WODITSCH4and KIRK-OTHMER5) may be consulted

as a useful source of information Literature closing date was spring 2019

In recommending the book to its readers, I like to acknowledge the advice byProfs Sebastian Hasenstab-Riedel and Christian Müller of the Free University Berlinand the help by Dr Anja Wiesner who assisted with the graphics Furthermore, I

am most grateful to my wife Dr Yana Steudel for many years of support and ation on the various editions of this book De Gruyter Publisher in Berlin supportedthis project from day 1, so that many people who worked with me all these yearsshould be acknowledged

cooper-Many colleagues, coworkers and students contributed to the success of thisbook with their comments and suggestions and I will be happy to hear from thereaders of this edition too

Berlin-Charlottenburg,

June 2019Ralf Steudel

1 Gmelin Handbook of Inorganic and Organometallic Chemistry – 8th edition, Springer, book series with 185 volumes (partly in English), published in 1936 –1995.

2 Ullmann ’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2006; many volumes and online edition.

3 Winnacker-Küchler: Chemische Technik, Vol 3: Anorganische Grundstoffe, 5th ed., Wiley-VCH, Weinheim, 2005.

4 K H Büchel, H.-H Moretto, D Werner (eds.), Industrial Inorganic Chemistry, 2nd ed., Wiley-VCH, Weinheim, 2008.

5 Kirk-Othmer (eds.), Encyclopedia of Chemical Technology, 5th ed., Wiley, New York, 2004; book series with 27 volumes.

Preface V

Part I: Chemical Bonds and Properties of Molecules

2 The Chemical Bond 11

2.1.2 The Ionization Energy Ei 13

2.1.3 The Electron Affinity Eea 15

2.1.4 Ionic Crystals and Ionic Radii 17

2.1.5 Lattice Energy and Lattice Enthalpy 19

2.1.6 Determination of Lattice Energies and Enthalpies 21

2.1.7 Significance of the Lattice Enthalpy 23

2.1.7.1 Complex Formation of Metal Halides 26

2.1.7.2 Weakly Coordinating Anions 27

2.1.8 Polarization of Anions by Cations 27

2.2 Molecular Geometry 30

2.2.1 Structure Determination 30

2.2.2 The VSEPR Model for Estimation of Molecular Geometries 322.2.2.1 Lone Electron Pairs 38

2.2.2.2 Single Electron Domains 40

2.2.2.3 Substituents of Differing Electronegativity 40

2.2.2.4 Multiple Bonds 41

2.2.2.5 Final Remarks on the VSEPR Model 41

2.3 Molecular Symmetry and Point Group Symbols 43

2.4.1 The Molecular Ion [H2]+ 49

2.4.1.1 The LCAO Approximation 50

2.4.2 The Molecule H2 57

2.4.2.1 The Cation [He2]+ 59

2.4.2.2 The Role of Antibonding Molecular Orbitals 59

2.4.3 Homonuclear Diatomic Molecules 60

2.4.4 Photoelectron Spectroscopy of Small Molecules 68

2.4.5 Heteronuclear Diatomic Molecules 70

2.4.5.1 General Rules for the Construction of MOs 72

2.4.6 Three-Atomic Molecules of D∞hSymmetry 73

2.4.6.1 The Molecule BeH 75

2.4.6.2 The Molecule CO2 76

2.4.7 Three-Atomic Molecules of C2vSymmetry 77

2.4.8 Four-Atomic Molecules of D3hSymmetry 80

2.4.9 Four-Atomic Molecules of C3vSymmetry 83

2.4.10 Five-Atomic Molecules 87

2.5 The Coordinate Bond 89

2.6 Hypercoordinate Molecules 93

3 VAN DERWAALSInteraction 101

3.1 The Dipole Effect 101

3.2 Induced Dipole Effect 104

3.3 The Dispersion Effect 104

3.4 VAN DERWAALSRadii 107

3.5 VAN DERWAALSMolecules 109

4.2.3 Why Is Oxygen a Gas and Sulfur a Solid? 122

4.3 The Internuclear Distance 124

4.4 The Valence Force Constant 127

4.6.2 Electronegativities (χ) 136

4.6.2.1 Thermodynamic Electronegativities by PAULING 136

4.6.2.2 Electronegativities according to ALLREDand ROCHOW 1384.6.2.3 Spectroscopic electronegativities according to ALLEN 1414.6.2.4 Group Electronegativities 142

4.6.3 The Dipole Moment 143

4.7 Electron Density Distribution in Molecules and Crystals 1474.7.1 Promolecule and Deformation Density 147

4.7.2 Halogen Bonding 149

Part II: Chemistry of the Non-Metals

5.5.2 Concentrated and Nonaqueous Acids 177

5.6 Hydrogen Bonds (H-Bonds) 181

5.6.1 Introduction 181

5.6.2 General Properties of Hydrogen Bonds 182

5.6.3 Experimental Detection of Hydrogen Bonds 183

5.6.4.2 Ice and Water 188

5.6.4.3 Gas Hydrates and Clathrate Hydrates 194

5.6.4.4 Ammonia and Amines 195

5.6.5 Theory of Hydrogen Bond Formation 198

5.7 Hydrogen Compounds (Hydrides) 202

5.7.5 Metal- and Alloy-Like Hydrides (Insertion Hydrides) 209

5.7.5.1 Bonding of Hydrogen in Alloy-Like Hydrides 211

6 Boron 215

6.2 Bonding Situation 216

6.2.1 LEWISAcidity and Adduct Formation 216

6.2.2 Coordination Numbers and Multiple Bonds 2196.2.3 Similarities and Differences to Other Non-Metals 221

6.3.1 Preparation of Elemental Boron 223

6.3.2 Solid-State Structures of Elemental Boron 2246.3.3 Bonding in Elemental Boron 226

6.4 Borides and Boron Carbides 228

7.4.2 Ionic Graphite Compounds 288

7.5 Carbon Black, Coal and Coke 291

7.7.1.3 Other Carbon Oxides 298

7.7.2 Sulfides, Selenides and Tellurides 299

7.7.3 Carbonic Acid and Carbonates 301

7.7.3.1 Carbonic Peracids 304

7.8 Carbon Nitrides 305

7.8.1 Hydrogen Cyanide and Cyanides 305

7.8.2 Binary Carbon–Nitrogen Species 307

8 Silicon and Germanium 309

8.2 Bonding Situation 310

8.3 The Elements Silicon and Germanium 318

8.4 Silicides and Germanides 322

8.5 Hydrides of Silicon and Germanium 325

8.5.1 Preparation of Hydrides 326

8.5.2 Reactions of Silanes and Germanes 328

8.6 Halides of Silicon and Germanium 329

8.6.1 Fluorides 330

8.6.2 Chlorides 331

8.6.3 Other Silicon Halides 333

8.7 Oxides of Silicon and Germanium 334

8.8.1 Silicic Acid and Siloxanes 338

9.3 Bonding in Nitrogen Compounds 372

9.3.1 Bond Enthalpies and Formation Enthalpies 376

9.4.8.1 Solubilities in Liquid Ammonia 393

9.4.8.2 Autodissociation of Liquid Ammonia 393

9.4.8.3 Ammonolysis 394

9.4.8.4 Solvated Electrons in Liquid Ammonia 394

9.4.8.5 Reactions of Solvated Electrons in Liquid Ammonia 3989.4.8.6 Solvated Electrons in Water 399

9.5 Halides and Oxohalides of Nitrogen 402

9.7.2 Nitric Acid (HNO3or HONO2) 417

9.7.3 Peroxonitric Acid (HNO4or HOONO2) 419

9.7.4 Nitrous Acid (HNO2or HONO) 419

9.7.5 Peroxonitrous Acid (HOONO) 421

9.7.6 Hyponitrous Acid (HON)2and Nitramide (H2NNO2) 422

10 Phosphorus and Arsenic 425

10.1 Introduction 425

10.2 Bonding Situation in P and As Compounds 425

10.3 Phosphorus and Arsenic as Elements 429

10.3.1 Production of the Elements 430

10.3.2 Modifications of Phosphorus and Arsenic 431

10.3.2.1 Structures of P and As Modifications 432

10.4 Hydrides of Phosphorus and Arsenic 437

10.4.1 Phosphane and Arsane 438

10.4.2 Diphosphane(4) 440

10.7 Diphosphenes and Phosphaalkynes 445

10.8 Halides of Phosphorus and Arsenic 447

10.8.1 Trihalides (EX3;E = P, As) 448

10.8.2 Tetrahalides (E2X4;E = P, As) 450

10.8.3 Pentahalides (EX5;E = P, As) 451

10.8.4 Strong LEWISAcids 453

10.9 Phosphoranes and Arsoranes 455

10.10 Oxides of Phosphorus and Arsenic 458

10.10.1 Phosphorus(III) Oxides 458

10.10.2 Phosphorus(V) Oxides 459

10.10.3 Phosphorus(III,V) Oxides 461

10.10.4 Arsenic Oxides 462

10.11 Sulfides of Phosphorus and Arsenic 463

10.12 Oxoacids of Phosphorus and Arsenic and Their Derivatives 46510.12.1 Oxoacids with One P Atom 465

10.12.1.1 Orthophosphoric Acid and Orthophosphates 467

10.12.1.2 Phosphonic Acid (H3PO3or HPO(OH)2) 469

10.12.1.3 Phosphinic or Hypophosphorous Acid (H3PO2) 470

10.12.1.4 Phosphinous Acid (H PO) and Hydroxophosphane (H POH) 470

10.12.2 Condensed Phosphoric Acids 470

10.12.3 Peroxophosphoric Acids 472

10.12.4 Thiophosphoric Acids 473

10.12.5 Halogeno- and Amidophosphoric Acids 473

10.12.6 Oxo- and Thioacids of Arsenic and Their Salts 47310.13 Phosphorus(V) Nitrides and Nitridophosphates 474

12 Sulfur, Selenium and Tellurium 521

12.1 Introduction 521

12.2 Bonding Situations and Tendencies in Group 16 522

12.3 Preparation of the Elements 525

12.3.1 Production of Sulfur 525

12.3.2 Production and Uses of Selenium and Tellurium 528

12.4 Allotropes of the Chalcogens 529

12.4.1 Sulfur 529

12.4.1.1 Thermal Behavior of Sulfur 530

12.4.1.2 Monotropic Sulfur Allotropes 534

12.4.2 Allotropes of Selenium and Tellurium 538

12.5 Homoatomic Chalcogen Cations 541

12.6 Chain Growth and Degradation Reactions 543

12.10.3 Lower Sulfur Oxides 563

12.11 Oxo-, Thio- and Halo-Acids of the Chalcogens 564

12.11.1 Introduction 564

12.11.2 Sulfurous Acid, H2SO3 566

12.11.3 Selenous Acid (H2SeO3) and Tellurous Acid (H2TeO3) 569

12.11.4 Sulfuric Acid (H2SO4) and Polysulfuric Acids (H2SnO3n+1) 56912.11.5 Selenic Acid (H2SeO4) and Telluric Acids [H2TeO4and

12.11.9 Dithionic Acid, H2S2O6 576

12.11.10 Dithionous Acid, H2S2O4 577

12.11.11 Disulfane Dithionous Acid, H2S4O4 578

12.12 Halides and Oxohalides of the Chalcogens 578

12.12.1 Introduction 578

12.12.2 Sulfur Halides 580

12.12.2.1 Fluorides 580

12.12.2.2 Lower Sulfur Fluorides 582

12.12.2.3 Chlorides, Bromides and Iodides 583

12.12.3 Sulfur Oxohalides 585

12.12.4 Selenium and Tellurium Halides 587

12.12.4.1 Fluorides 587

12.12.4.2 Selenium Chlorides and Bromides 587

12.12.4.3 Tellurium Chlorides and Bromides 588

12.12.4.4 Polymeric Tellurium Halides with Te‒Te Bonds 589

12.12.4.5 Halochalcogenate Anions 590

12.12.4.6 Bonding in Selenium and Tellurium Halides 591

12.13 Sulfur–Nitrogen Compounds 591

13.4.4 Applications of Fluorine Compounds 612

13.4.4.1 Liquid Hydrogen Fluoride 612

13.4.4.8 Liquid Crystals 615

13.4.4.9 Fluorinated Pharmacologically and Agrochemically Active

Substances 615

13.4.5 Bonding Situation in Fluorides 616

13.4.6 Stabilization of Low Oxidation States 617

13.4.7 Stabilization of High Oxidation States 619

13.5 Chlorine, Bromine and Iodine 620

13.5.1 Preparation and Properties of the Elements 620

14.3.2.2 Fluoride Transfer Reactions 659

14.3.3 Oxides and Oxosalts of Xenon 661

14.3.4 Xenon Oxyfluorides 663

14.3.5 Other Xenon Compounds 664

14.4 Compounds of Other Noble Gases 666

14.4.2 Krypton 667

14.5 Electronegativities of Noble Gases 667

14.6 Bonding Situation in Noble Gas Compounds 669

14.6.1 Diatomic Molecules and Ions 669

14.6.2 Polyatomic Molecules and Ions 669

14.6.3 Existence and Nonexistence of Noble Gas Compounds 672Index 675

Chemistry is a central science because all life-sustaining processes are based on ical reactions, and all items that we use in everyday life consist of natural or artificialchemical compounds.“Chemistry is also a fantastic world populated by an unbeliev-able number of nanometric objects called molecules, the smallest entities that havedistinct shapes, sizes and properties Molecules are the words of matter Indeed, most

chem-of the other sciences have been permeated by the concepts chem-of chemistry and the guage of molecules Like words, molecules contain specific pieces of information thatare revealed when they interact with one another or when they are stimulated by pho-tons or electrons In the hand of chemists, molecules, particularly when they are suit-ably combined or assembled to create supramolecular systems, can play a greatvariety of functions, even more complex and cleverer than those invented by Nature.”1

lan-However, chemistry is not only part of the natural sciences but also a big businessemploying millions of people The global sales of chemical and pharmaceutical indus-tries totaled the enormous amount of 4,751,372,000,000 Euro in 2017, with China con-tributing the biggest share followed by Europe, the USA and Japan (Table 1.1) InEurope, Germany is by far the biggest producer with BASF at Ludwigshafen (Germany)

as the largest chemical site worldwide shown on the cover

Table 1.1: Sales of the chemical and pharmaceutical industries in 2017

worldwide and in different regions and countries (in billion Euro), ordered

by decreasing percentages (other countries: <100 billion Euro each).2

Thenonmetallic elements and their compounds are the basis of many cal and modern industrial processes,3resulting in products that enable mankind tolive a comfortable and healthy life, at least in the developed countries but increas-ingly so also in the developing part of the world For example, fertilizers based onammonia guarantee a sufficient global food production to nourish the 7.7 billionpeople on the Earth (2019) – and probably even more in the foreseeable future.Without the invention of synthesis of ammonia from the elements, billions of peo-ple had probably died from starvation during the last 100 years Therefore, not lessthan three NOBELprizes have been awarded to chemists and engineers for the inven-tion, the industrial development and the mechanistic explanation of the synthesis

classi-of ammonia as an archetypical example classi-of heterogeneous catalysis (see Chapter 9)

In the following, let us make a short excursion to the main groups of thePeriodicTable containing 23 nonmetallic elements to demonstrate their importance for mod-ern civilization by selected examples of compounds, products and processes

Hydrogen gas is not only an important reducing, hydrogenating and tion agent in chemistry but increasingly serves as a fuel and energy storage materialfor environmentally friendly power generation and mobility Hydrogen-powered fuelcells in cars, trucks and trains have been developed for future transportation in citiesplagued by air pollution from the combustion of fossil fuels Therefore, the cost-efficient production of elemental hydrogen from water using renewable energy sour-ces such as sunlight and wind is presently a research topic of utmost importance,especially the catalytic water splitting by solar radiation Elemental hydrogen is alsoused to propel rockets to transport heavy cargo into space As an element, hydrogen is

desulfuriza-a key component of desulfuriza-all orgdesulfuriza-anisms, for exdesulfuriza-ample, in the form of wdesulfuriza-ater, cdesulfuriza-arbohydrdesulfuriza-ates,fats, amino acids, proteins, peptides, enzymes and vitamins In this context, intermo-lecular hydrogen bonds are of pivotal importance in chemistry, biology and medicine.The three-dimensional structures of virtually all biological molecules such as proteins,enzymes and DNA heavily depend on hydrogen bonds

Deuterium (2H or D) as the next heavier isotope of hydrogen shows twice theatomic weight, which accounts for unusually pronounced isotopic differences inphysical and chemical properties Heavy water D2O– first prepared in 1933 by con-tinued electrolysis of water– made the construction of the first nuclear reactor pos-sible in which it served as a neutron moderator and coolant

Boron compounds are not too visible in daily life, but chemists appreciateevery day the chemical and thermal resistance of borosilicate glassware in the labo-ratory Similarly, boron nitride and boron carbide are high-performance ceramic ma-terials and serve, inter alia, as neutron absorbers in nuclear reactors On the otherhand, the large absorption cross section of the isotope10B for neutrons is used forcancer therapy by neutron irradiation Furthermore, hydroborates are important

3 www.essentialchemicalindustry.org.

reducing agents, and boranes as well as boronic acids are valuable reagents in ganic synthesis Finally, perborates are known as powerful bleaching agents in de-tergents at home.

or-The inorganic chemistry ofcarbon comprises the allotropes of the element aswell as carbon compounds lacking any CH bonds Graphite as the thermodynamicallystable modification is most important, but more spectacular are diamonds, graphene,fullerenes and carbon nanotubes The latter three are high-tech materials, which areextensively explored for applications based on their astonishing physical and chemi-cal properties Graphite is a metallic electrical conductor Carbon materials of thistype are utilized as anodes in lithium-ion batteries, which power mobile phones, lap-tops and tablets as well as millions of electric cars on the streets In addition, carbon-based anodes are applied in large-scale industrial processes such as electrolyticalproduction of fluorine, chlorine, sodium, aluminum and many other elements.Diamonds do not only serve as “girls’ best friends,” but they make high-performance cutting and drilling tools more durable owing to their unparalleledhardness The discovery of the new class of carbon allotropes called fullerenes re-sulted in the synthesis of numerous derivatives with amazing structures and prop-erties Carbon nanotubes and graphene materials with their unique electronicproperties are also investigated with high intensity, and a steadily increasingnumber of related papers are published each year The European Union supportsthis work with 100 million Euros per year The invention of carbon fibers madecars and aircrafts more efficient due to their reduced weight And composite mate-rials based on carbon nanotubes allow the construction of ever-larger blades forwind generators owing to their enormous tensile strength

On the dark side, the rising CO2concentration in Earth’s atmosphere is of greatconcern since it is held responsible for global warming and climate change due tothe greenhouse effect, together with other“greenhouse gases” such as N2O, NF3and CH4 Thus, the decarbonization or defossilation of the world’s economy byswitching from fossil fuels to renewable energy sources is one of the most importantchallenges of this century, and chemists are expected to contribute significantly tothis energy revolution

Silicon is the second most abundant element in Earth’s crust and has been used

in the form of silicates since ancient times to make glass and ceramics Today, purity elemental silicon as a semiconductor is a major component in electronic andphotovoltaic devices, which changed our civilization dramatically during the last

high-50 years The popular expression Silicon Valley is an indication for this electronic lution, which started more than 100 years ago with the first extensive fundamental in-vestigation of the silanes and chlorosilanes by ALFRED STOCK Silicones (RSiO)n asinorganic polymers are another multibillion business owing to their numerous applica-tions in households, construction business and industry They are based on thefirst direct synthesis of methyl chlorosilanes independently developed in 1941 by

revo-RICHARDMÜLLERin Germany and EUGENEROCHOWin the USA Furthermore, glass fibers

for optical data transmission with high speed and high volume are made of silicon oxide Silicate or silicon carbide fibers form the basis of valuable composite materials.Nitrogen is one of the most important elements in living organisms in which itoccurs in amino acids as components of peptides and proteins The bases of RNA alsocontain nitrogen atoms, essential for pairing to DNA via NH····N and NH····O hydrogenbonds This nitrogen originates from ammonium ions in soil, which are supplemented

di-to some extent through the natural nitrogen cycle but in modern agriculture mostly byfertilizers containing ammonium salts, urea or nitrates Artificial nitrogen fertilizers areproduced from ammonia, which in turn is obtained from dinitrogen from the atmo-sphere and dihydrogen in the HABER-BOSCHprocess Since 1950 the world’s production

of ammonia has increased faster than the world’s population On the other hand, gen oxides (NOx) are formed from air in all combustion processes including Diesel en-gines and are blamed for health problems in urban areas Thus, chemists andengineers work to remove these compounds from exhaust gases by catalytic decompo-sition On the other hand, NO functions in tiny concentrations as a signaling molecule

nitro-in mammals and is responsible for the regulation of blood pressure, blood coagulationand immunity This discovery has been honored by the NOBELprize in medicine in

1998 Less well known is the use of high-energy fuels such as hydrazine and its tives in spacecrafts to propel them into space and to distant planets and comets.Phosphorus is one of those highly oxophilic elements that occur in Nature exclu-sively in the form of oxosalts such as phosphates Animals need calcium phosphate tostabilize their bones and teeth, and adenosine triphosphate (ATP) is the dominatingenergy storage compound of cells in all organisms including mammals and humans Aresting person converts ca 40 kg of ATP per day and even 0.5 kg per minute duringhard work The exothermic hydrolysis of ATP to the diphosphate (ADP) and hydrogenmonophosphate under physiological (nonstandard) conditions (300 K, pH = 7.4) is asso-ciated with an enthalpy change of approximately−65 kJ mol–1; the liberated energy isused in muscles and other organs Therefore, ATP must be permanently re-synthesized

deriva-by cellular respiration with energy delivered deriva-by the oxidation of food components (or

by sunlight in plants) Consequently, phosphates play a central role as fertilizers andfood additives in farming as well as in nutrition In addition, there are numerous nitro-gen- and phosphorus-containing herbicides and insecticides for pest control in thefarming business

Arsenic and its compounds are infamous for their toxicity This seemingly pleasant property is in fact applied to good advantage in medicine For example,arsenic trioxide is used to treat a special form of leukemia, and certain organoar-senic compounds help to treat sleeping sickness Chemotherapy in medicine wasfirst established by PAUL EHRLICH4 and coworkers who discovered the beneficial

un-4 P Ehrlich (185un-4 –1915), German physician and physiologist (N OBEL prize in medicine in the year 1908).

action of Salvarsan and Neosalvarsan (RAs)n(n = 3–5) to treat syphilis in 1910–1912(both were replaced for this indication by penicillin in the 1940s) Today, arsenic is

a valuable alloying component in metals; it is also needed to produce tors such as gallium and indium arsenides

semiconduc-Oxygen is the most abundant element in the accessible parts of planet Earth

It occurs as silicates, phosphates, sulfates and oxides in the upper crust, as water

in the oceans, lakes, rivers and clouds, as O2(and O3in minor quantities) in theatmosphere, as well as in numerous organic compounds of what is called biomass.While O2from photosynthesis of green plants is the“elixir of life” for animals andhumans, the thin ozone layer formed by UV photolysis of O2in the stratosphereprotects all organisms living on the continents from the destructive hard UV radi-ation of the Sun Thus, the formation of the ozone layer was a precondition for life

to migrate from the oceans (where it probably originated) to the dry surface of theEarth On the other hand, reactive oxygen species (ROS) such as radicals and per-oxides produced during respiration processes are harmful to human health andneed to be destroyed by antioxidants such as certain vitamins or possibly also byphenolic components in red wine and chocolate In industry, huge amounts of O2are required to oxidize inorganic and organic compounds in numerous differentproduction processes Furthermore, O2is applied on a large scale to reduce thecarbon content of pig iron to turn it into steel

Sulfur compounds have probably played a crucial role in the origin of life nearly

4 billion years ago Before the atmosphere acquired its high oxygen content oftoday, it contained substantial concentrations of H2S and this gas together with FeSand hot water from hydrothermal vents on ocean floors has probably been essentialfor the synthesis of the first organic molecules by reduction of CO2(the biologicalevolution occurred under anoxic or extremely hypoxic conditions, that is, low in ox-ygen, most of the time) For all organisms, sulfur is an essential element Therefore,coal and crude oil originating from ancient plants contain up to several percent ofsulfur Today desulfurization of crude oil and of sour natural gas yields enormousamounts of H2S, which is oxidized to elemental sulfur as an essential raw materialfor large-scale production of sulfuric acid and many other chemicals including fertil-izers Sulfur and its compounds are also needed for production of tires for cars andtrucks as well as other rubber materials by the vulcanization process Future electriccars may even be powered by lithium-sulfur batteries, which have a much higherpower capacity than today’s lithium-ion batteries and would enable a larger range

of operation

Selenium is a rather rare element on the Earth, but it is essential for life Morethan 20 natural selenoproteins have been identified as redox regulators in animals.Therefore, fertilizers and animal feed are supplemented with traces of seleniumcompounds in areas where farmland is Se-deficient In the electronics industry, se-lenium and certain selenides such as CuInSe2serve as semiconductors in thin-filmphotovoltaic cells

Fluorine is the most reactive of all chemical elements It was first prepared in

1886 by HENRI MOISSAN in France who received the NOBEL prize in chemistry ofthe year 1906 for this ground-breaking work Today fluorine-containing groups arepresent in many pharmaceutical drugs and in agrochemicals to make them more li-pophilic and resisting to metabolic and other degradation reactions for a longer pe-riod of time Fluorine renders high-performance materials such as Teflon andNafion chemically resistant

Elemental fluorine is used on a large scale to produce UF6, which is employedfor the enrichment of the uranium isotope 235U from the natural mixture with

238U Enriched uranium is needed for nuclear reactors, which produce energy fromthe spallation of the235U nucleus by neutrons In our daily life, fluorides in tooth-paste protect our teeth from caries, while Li[PF6] serves as an electrolyte in com-monplace lithium-ion batteries Molten Na3[AlF6] is most important as electrolyte inaluminum production from Al2O3 by electrolysis And even the theory of chemicalbonding has profited from the high reactivity of elemental fluorine since the firstneutral noble gas compounds to be prepared were XeF2and XeF4(made from theelements), triggering a change of paradigm in chemical thinking beginning in the1960s

Chlorine gas is one of the most important reactants in industry although it dom shows up in the final products for sale Many organic chemicals are made viareactive chlorine-containing intermediates Cl2 is produced in huge quantities byelectrolysis of aqueous NaCl and the efficiency of this energy-intensive process hasrecently been improved by 30% using special oxygen-consuming electrodes Thus,even well-established production processes can still be dramatically modernized tosave energy Chlorine compounds are needed to synthesize pharmaceutical drugs,dyestuffs, fertilizers, insecticides, plastics, fibers and modern functional materials

sel-On the other hand, chlorofluorocarbons (CFCs) are harmful to the protective ozonelayer in the stratosphere For the discovery of this effect, the NOBELprize in chemis-try for the year 1995 has been awarded to PAULCRUTZEN, MARIOMOLINAand FRANK

SHERWOODROWLAND In the meantime, the production of CFCs has been ally banned and other chemicals have taken their place as refrigerants, propellantsand solvents

internation-The noble gases, originally discovered spectroscopically by physicist, are nolonger as noble as once thought: the number of molecular and salt-like xenon com-pounds exceeds 100 and is growing from year to year KrF2as a rare example of akrypton compound is also studied extensively Highly unusual species such as[Xe2]+ und [AuXe4]2+have been isolated as salts The main applications of noblegases, however, are still of a physical nature: Gaseous He is used for cooling of nu-clear reactors and to lift balloons and airships In liquid form, it is employed ascoolant for superconducting magnets, for example, at the large hadron collider(LHC) in Switzerland and for cooling of man-made satellites orbiting the Earth.Besides, He, Ne, Ar and Kr are used to produce coherent radiation in gas lasers; Ne,

Kr and Xe are applied in fluorescent and incandescent lamps, and– most tantly for chemists– Ar serves as protective gas in glove boxes in the laboratory aswell as in industry.

impor-Very recently, the“oldest molecule of the universe” formed about 100,000 yearsafter the Big Bang was discovered by astrophysicists At that time the original gas andplasma cloud had cooled down enough that He existed mainly as neutral atoms whilehydrogen existed still mostly ionized as free protons Since He has a high proton affin-ity, the ion [HeH]+is thought to be the first molecular entity ever formed in the uni-verse In 2018 [HeH]+ has been observed by its emission spectrum in the planetarynebula NGC7027 using the high-flying telescope Sofia built into an Airbus aircraft.5

InPart II of this textbook the preparation, structures, properties and applications

of the nonmetallic elements and their most important compounds are described in tail, starting with hydrogen and then following the order of the Periodic Table Thechemical literature up to spring of 2019 has been considered, and many references torecent reviews allow further reading on special topics To fully understand the struc-tures and reactivities of molecules and materials, however, a good knowledge of theunderlying theory of chemical bonding is necessary This theory has originally beendeveloped on the basis of empirical observations, for example, spectroscopic and ther-modynamic data Today theoretical concepts are not only expected to explain well-established facts but to provide reliable predictions of the structures and properties ofstill unknown substances Only on this basis, new materials with specific functionscan be developed for defined high-tech applications

de-Therefore, inPart I of this book the theoretical concepts necessary for the derstanding of nonmetallic systems are discussed In this context, it is not alwaysnecessary to turn to quantum mechanics, since simple model descriptions of mole-cules are often sufficient to rationalize and classify the enormous amount of today’sempirical knowledge in chemistry These model descriptions are also helpful inteaching regardless whether they are true or not Of course, the limits of the particu-lar model have to be respected and they cannot be used to explain Nature In thissense, the basic concepts of chemical bonding theory are outlined in Chapter 2, the

un-VAN DERWAALSinteraction is explained in Chapter 3 and the empirical properties ofcovalent bonds in molecules such as bond energies, bond lengths, bond polaritiesand intermolecular interactions are discussed in Chapter 4

Textbooks are mainly dealing with the present knowledge but only rarely withstill unknown substances, although they may play an important role in the future.Chemical Abstract Service (CAS) in Columbus (Ohio) has already registered morethan7·107compounds, but most of the theoretically possible molecules have prob-ably not been synthesized or discovered yet Combining an increasing number ofdifferent chemical elements in one molecule or one material opens the possibility to

5 R Güsten et al., Nature 2019, 568, 357.

create completely new functions, for example, for catalysis or medical applications.

To predict the physical, chemical and functional properties as well as the potentialusefulness of such systems requires a good understanding of the present-dayknowledge of atoms, molecules, bond properties and theoretical as well as empiri-cal concepts and relationships Therefore, the corresponding theoretical fundamen-tals will be outlined in Part I of this book, followed by the larger Part II covering thesynthesis, structures and reactions of the most important inorganic compounds ofthe nonmetallic elements

The theoretical treatment of chemical bonding in molecules, ions and crystals isamong the most challenging problems in chemistry In particular, if a readily acces-sible interpretation of experimental observations or of the results of theoretical cal-culations is sought, the use of appropriate models cannot be avoided Since themicroscopic world is dramatically different from our macroscopic experience, ap-proximations and idealizations are necessary to elucidate the behavior of atomsand molecules After all, physics does not describe the real world but only our knowl-edge of the real world.

The situation is even less satisfying when having a closer look at the generalproperties of electrons, which are responsible for the bonds between the nuclei

of atoms and thus determine most of chemistry Electrons are practically sionless particles (diameter≤ 10−18 m) with a certain charge, a certain mass and

dimen-a spin The spin is dimen-an dimen-abstrdimen-act qudimen-antum-mechdimen-anicdimen-al property of electrons dimen-andshould not be identified with the rotation of a particle, although it has the samedimension as an angular momentum These particle-like properties follow fromspectroscopic observations and from experiments regarding the behavior of elec-tron beams in electric and magnetic fields On the other hand, electron beamsbehave as a sequence of small wave packets and thus undergo scattering andinterference, which are typical properties of electromagnetic waves (e.g., pho-tons) In other words, in the atomic world there are complementary properties.One may ask: How can a seemingly zero-dimensional point have a mass, acharge and a spin? Where does this mass come from? According to quantumfield theory all elementary particles are excitations of omnipresent fields, one foreach particle.1

Despite this ambiguity, theoretical calculations based on the SCHRÖDINGERtion and using four quantum numbers for each electron predict the behavior of elec-trons in atoms and molecules with reasonable accuracy and even provide more orless reliable enthalpies of real and hypothetical chemical reactions This area ofcomputational chemistry has become increasingly popular in recent times Mostchemical publications on inorganic molecules these days contain the results ofquantum-chemical calculations to some extent

equa-A theory is useful if it explains a large body of observations with a minimum

of parameters and assumptions In addition, it should correctly predict the erties, structures and reactions of yet-to-be prepared compounds We need tokeep in mind, however, that any theory exists only in our brain and this also

prop-1 Theoretical physicists believe that the mass of elementary particles is the result of their tion with a background field first proposed by the British theoretical physicist PETER HIGGS (HIGGS field); he was born in 1929 and received the NOBEL prize in physics for the year 2013.

interac-applies to the components of the theory, for example, atomic and molecular tals (MOs) as well as bonds In this sense, a theory is just a working hypothesis,which cannot be proven as true, but can always be falsified and replaced by an-other theory A good example is the first model of the hydrogen atom published

orbi-by NIELSBOHRin 1913 At that time, BOHR’s theory was considered a revolutionarydevelopment in atomic physics and consequently he was awarded the NOBELprize

in physics Only a few years later the quantum-mechanical model of the hydrogenatom replaced BOHR’s theory, and other atomic physicists received the NOBEL

prize.2

A chemical bond is a result of the accumulation of negative charge density inthe region between a pair of (positively charged) nuclei, which is sufficient to bal-ance the forces of electrostatic repulsion In chemistry, it is common to discuss thephenomenon of chemical bonds at different levels of accuracy, depending on thequestions to be answered A satisfactory theory of the chemical bond should be able

to provide answers to the following fundamental questions:

Why do atoms combine to form molecules and crystals?

Why do they do so in certain ratios, and often in several ratios, for example,

NO, NO2, N2O, N2O3, N2O5?

Why do molecules and crystals have discrete structures?

Why do molecules react with each other the way they do?

To answer these and related questions, it is appropriate to consider distinct bondtypes:

(a) the ionic bond,

(b) the covalent bond including coordinate or dative bonds and

(c) theVAN DERWAALSinteraction

From the structures of several hundred thousand molecules determined by X-raycrystallography, electron diffraction, vibrational, nuclear magnetic resonance (NMR)and microwave spectroscopy, we have learned that there is a continuous transitionbetween these three fundamental bond types In most compounds more than one ofthese idealized types contributes to the overall bonding situation We begin our dis-cussion with the model of the ionic bond, which is built exclusively on electrostaticforces without any quantum mechanics and therefore most easily understood

2 NOBEL prizes in physics have been awarded to NIELS BOHR (1922), LOUIS V DE BROGLIE (1929), WERNER HEISENBERG (1932), ERWIN SCHRÖDINGER (1933), WOLFGANG PAULI (1945) and MAX BORN (1945) WALTER KOHN and JOHN POPLE received the NOBEL prize in 1998 for the development of algorithms and software for quantum-chemical calculations.

2.1 The Ionic Bond

2.1.1 General

A huge number of compounds crystallize in ionic structures consisting of periodic, ular, three-dimensional arrays of cations and anions The ions may be monoatomic orcomplex, that is, derived from molecules:

reg-Li+ and H− in LiH ½NO+ and HSO

2.1.2 The Ionization Energy Ei

The first ionization of a gaseous atom A according to the equation

i

requires an enthalpyΔH°, which, for historic reasons, is called ionization energy Ei

and is always positive.3For this process it is assumed that the least tightly boundelectron is removed from the atom (taken from the highest occupied atomic orbital).The values of Ei depend strongly on the position of the atom A in the PeriodicTable Metal atoms are most easily ionized, while noble gas atoms have the highest

Ei values Ionization energies of neutral atoms range from 4 to 25 eV or 400 to

en-The large ionization energies of the noble gases are of enormous importance; theyare caused by the large effective nuclear charge Zeffacting on the valence electrons(VEs) of these atoms with their filled s and p levels According to empirical rules origi-nally published by JOHNSLATER, the Zeffvalues can be estimated for each electron con-figuration taking the partial shielding of the nuclear charge by the other electrons intoaccount For the elements of the first period, the following data are obtained for theVEs in the highest occupied orbitals:4

There is a relationship between the value of the first ionization energy Ei(1) and theabsolute energyε of the orbital from which the electron has been removed.5Thesetwo energies are often identified with each other, an approximation that is known as

KOOPMANS’ theorem For example, Eiof the carbon atom is 11.3 eV, while the energy

of the 2p orbital has been calculated as−10.7 eV The difference between these twoenergies, using absolute values, is caused by the simultaneous rearrangement of

K

Zn

Ga Rb

Ra (eV)

the remaining electrons during ionization (relaxation) since the effective nuclearcharge increases and the interelectron repulsion decreases on ionization Forlithium Ei(1) = 5.39 eV and ε(2s) = −5.34 eV, while for neon Ei(1) = 21.56 eV andε(2p) = −23.14 eV.

As shown above, the ionization energy of oxygen (13.6 eV) is– somewhat pectedly– smaller than Eiof nitrogen (14.5 eV) In the N atom and in the cation O+there are three electrons of equal spins occupying the 2p level resulting in maximalexchange interaction, which stabilizes this configuration: the three electrons occupyone orbital each, thus avoiding identical space segments and diminishing the

unex-COULOMB repulsion Their equal spins minimize the PAULI repulsion at the sametime.6In other words, if the ionization results in a half-filled electronic (sub)shell,the required ionization energy is slightly lower than expected

Similarly, the lower ionization energy of the boron atom compared to ing beryllium is caused by the fact that the removed electron comes from a 2p or-bital in the case of boron but from the lower 2s level in the case of beryllium.The second ionization energy Ei(2) of an atom according to the equation

neighbor-A (g.)+ A (g.) +2+ e–

is always considerably larger than Ei(1) as the electron must now be removed from

a particle bearing a positive charge already For example, Ei(2) of the carbon atom

is 24.4 eV, although the removed electron comes from the same 2p level as in thefirst ionization This means that the orbital energies of the cation C+are much lowerthan those of the neutral atom, which is readily explained by the higher effectivenuclear charge Ei(2) also varies periodically with the atom number, but the maximaare now observed for the ions with noble gas electron configuration, for example,

Na+and K+ The curve in Figure 2.1 would thus be shifted to the right by one atomicnumber, and in a similar manner for the third ionization energy Ei(3)

2.1.3 The Electron Affinity Eea

Most gaseous nonmetallic atoms B can capture an electron in an exothermic reaction:

B(g.) + e B (g.) H° =Eea

6 According to the PAULI exclusion principle, electrons of the same spin behave as if there were

a repulsive force acting between them Consequently, two electrons of equal spin cannot cupy the same space or orbital at the same time Since only two spin states are possible and since quantum numbers differ by one unit these states are characterized by the spin quantum numbers +½ and −½.

oc-The electron affinity Eeais taken as positive if energy is released on electron capture.7For stable anions, Eearanges from 0 to 3.6 eV or 0 to 350 kJ mol–1(Figure 2.2) Incontrast to the ionization energies, electron affinities do not show a periodic trendwith atomic number The effect of filled or half-filled electronic (sub)shells, however,

is far more pronounced: Noble gases, nitrogen and some metal atoms do not formstable anions and their electron affinities are therefore zero (or even negative).8

The electron affinity of an atom is identical to the ionization energy of its mono-anion:

B–(g.) B(g.) + e– Ei(B–) =Eea(B)

For example, Eeaof the carbon atom is 1.27 eV, and the ionization energy of theanion C– (1.27 eV) is consequently much smaller than that of the neutral carbonatom (11.3 eV) The formal addition of a second or third electron to an atom is al-ways strongly endothermic, that is, Eea(2) and Eea(3) are always negative and can

F

Cl Br I At

Po Te Se S

O

Bi Sb As P

Kr 0

Ar 0

Ne 0

He 0

I 3.06

Te 1.97

Sb 1.05 1.25

In 0.30

Br 3.36

Se 2.02 (–4.35)

As 0.80

Ge 1.24

Ga 0.30

Cl 3.61

S 2.08 (–6.11)

P 0.74

Si 1.38

Al 0.46

F 3.40

O 1.46 (–8.08)

N –0.07 (–8.29)

C 1.27

B 0.28

H

F O

8 J Furtado, F De Proft, P Geerlings, J Phys Chem A 2015, 119, 1339 K D Jordan, V K Voora,

J Simons, Theor Chem Acc 2014, 133, 1.

only be determined by thermochemical cycles (see below) since such ions do notexist except (formally) in crystals The Eea values in Figure 2.2 demonstrate thations with noble gas electron configuration are particularly stable As a conse-quence, the halogen atoms show the largest electron affinities.

Here it should be stressed that multiple charged anions such as O2–and S2–butalso [CO3]2–, [SO4]2–and [PO4]3–do not exist as isolated ions, for example, in the gasphase They would immediately lose one or two electrons by a process called spon-taneous electron autodetachment to form mono-anions Small complex anions mayalso dissociate exothermically into mono-anions by what is referred to as COULOMB

explosion.9Therefore, any experimental observation of such ions in the gas phase

is impossible The negative electron affinities listed in Figure 2.2 have a more formalcharacter and have been derived from thermodynamic cycles, which will be dis-cussed in Section 2.1.6 In condensed phases, on the other hand, all anions are sur-rounded by cations or polar solvent molecules, which reduce the anionic chargesand thus stabilize the multiple charged anions.10 The real charges of cations andanions in crystals and solutions are known only in a few cases and strongly depend

on the volume assigned to these ions

2.1.4 Ionic Crystals and Ionic Radii

Rock salt (NaCl) will be taken as the standard example of an ionic crystal sincethe halides of all alkali metals except cesium adopt this structure type.11Preciseinternuclear distances can be determined from X-ray diffraction on a single crystalalthough the diffraction takes place at the electrons rather than at the nuclei.Since the highest electron density is near the nuclei, their approximate positionscan nonetheless be determined.12Sodium chloride has a face-centered cubic unitcell The unit cell is the smallest array containing all the symmetry elements ofthe crystal, which can formally be constructed by periodic translational repetition

of the unit cell in all three dimensions The infinite set of translational vectorsforms the crystal lattice The positions of the ions in the rock salt structure areshown in Figure 2.3

9 A I Boldyrev, J Simons, J Phys Chem 1994, 98, 2298 R Janoschek, Z Anorg Allg Chem 1992,

616, 101 Only much larger particles such as the fullerene C 60 can accommodate more than one ditional electron in the gas phase.

ad-10 See for instance: S Sasaki et al., Acta Cryst A 1980, 36, 904.

11 The cesium halides CsX (X=Cl, Br, I) crystallize in cubic body-centered structures with tion numbers of 8 for all ions (each ion is at the center of a cube of counter-ions).

coordina-12 Diffraction of neutrons takes place at the nuclei, and for the exact determination of positions especially of hydrogen atoms with their large vibrational amplitude and low electron density neu- tron diffraction is the preferred method.

From precise X-ray diffraction data, it is possible to calculate, in addition to ion cations, the total electron density distribution in a crystal, measured in e a0–3 or

lo-e Å–3(1 e a0 –3= 6.749 e Å–3) For rock salt, the result is shown in Figure 2.3 in theform of a contour map of isodensities for a segment of the shown (110) plane Theelectron densities decrease from the center of the atoms to the outside– strongly atfirst, then less so On the shortest connection line between neighboring cations andanions, the density drops to a minimum of 0.2 e Å–3(1 Å = 100 pm) This point ofalmost zero density can be considered as the boundary of the two contacting ions

of opposite charge, with the ionic radii defined as the distances from the sponding nuclei to this point Integration of the electron density over the sphericalvolumes limited by the ionic radii yields 10.05 electrons for Na+and 17.70 electronsfor the Cl−ion, compared to 10 and 18 for pseudo-Ne and pseudo-Ar configurations,respectively The“missing” 0.25 electrons can be expected in the interstices of thespherical packing, which were not considered in the integration (see Figure 2.3).Thus, the structure consists of ions rather than of neutral atoms

corre-For our future discussions we assume that atomic ions in crystals are mately spherical and rigid and have a characteristic radius (crystal radius).However, the radii derived from electron density maps are not constant but depend

approxi-on the particular crystal structure and the coordinatiapproxi-on numbers of the iapproxi-ons.13The

100 pm

–

–

– –

– – –

+

–

Figure 2.3: Right: Face-centered cubic packing of sodium cations and chloride anions in rock salt Left: Experimental electron density distribution in the (110) plane of NaCl The larger Cl−ions have

a maximum charge density of 55.8 and the smaller Na+ions of 29.8 e Å–3(1 Å = 100 pm).

13 I D Brown, Encycl Inorg Chem 2005, 1, 446 A Shannon, Acta Cryst A 1976, 32, 751.

variation of ionic radii (in pm) of different ions with the number of electron shellsand ionic charges at a constant coordination number of 6 is demonstrated by thefollowing examples (the corresponding isoelectronic noble gas is given at the top ofeach column):

As expected, ionic radii increase with the number of electron shells but decreasewith increasing positive charge and increase with increasing negative charge Thelatter is the consequence of the varying effective nuclear charge, which attracts theVEs as well as of the varying electron–electron repulsion In the following series ofisoelectronic ions, the radii increase from cations to anions:

Ca2+< K+< Cl−< S2−

The concept of pure ionic bonding is an idealized approximation, which is nearlyfullfilled in crystals of two elements with a large electronegativity difference Sincethe alkali metals have the lowest and fluorine and oxygen the highest electronega-tivities (Section 4.6.2), the fluorides and oxides of these metals are probably thebest examples for this bond type As will be seen in the following sections, how-ever, certain covalent contributions by mutual polarization of the ions as well as

VAN DERWAALSinteractions also contribute to the stability of ionic crystals

2.1.5 Lattice Energy and Lattice Enthalpy

The stability and properties of ionic compounds are derived from the lattice energyand enthalpy for which the symbols UoandΔlatH° are used, respectively The latticeenthalpy is defined as the enthalpy liberated when equivalent molar amounts ofgaseous cations and anions are combined from infinite distance to form a singlecrystal:

A (g.) + B (g.)+ AB(s.) ° < 0

SinceΔlatH° is liberated from the system, it always carries a negative sign (a“largelattice energy” means a highly negative value of ΔlatH°) Lattice energy and enthalpyare related by the contribution of the volume change:

O–:  S–:  Se–:  Te–: 

ΔlatH = ΔlatU+ pΔV (2:1)Only at 0 K,ΔlatH° andΔlatU° are identical, but nevertheless the term“lattice en-ergy” is often used for either one in the physicochemical literature This is somehowjustified since the absolute values ofΔlatH° andΔlatU° differ just slightly at 25 °C.The lattice energyΔlatU° consists of several components, which are summarized forthree metal halides in Table 2.1.

The largest contribution arises from the electrostatic attraction between ions ofopposite charge and repulsion between ions of equal charge (COULOMB interac-tion) In addition,VAN DERWAALSattraction works independently of charge (seeChapter 3) Note that this component contains some covalent bonding, the abso-lute strength of which is inversely proportional to the electronegativity difference

ΔχPbetween the two elements (PAULING values):ΔχP= 2.3 (NaCl) > 1.9 (CsI) > 1.3(AgCl)

At equilibrium, attractive forces are balanced by repulsion arising from mutualinterpenetration of the electronic clouds of neighboring ions as well as from the in-teraction of equally charged next neighbors but one Like atoms, ions have no fixedboundaries In the formation of NaCl crystals to reach an internuclear distance of281.4 pm, the electron clouds of cations and anions mutually penetrate slightly, re-sulting in some contraction of the ions This increases the potential energy of theions and reduces the lattice energy as shown in the second line of Table 2.1 (BORN

repulsion)

The fourth component of lattice energy, the zero-point energy, is the vibrationalenergy of the ions, which remains even at 0 K This energy can be calculated fromthe lattice vibration frequencies obtained from infrared (IR) or RAMANspectra Thezero-point energy has a relatively small effect on the lattice energy; therefore, it isoften ignored

Table 2.1: Components of the lattice energy of three metal halides (in kJ mol–1) The

decomposition of the total lattice energy into components rests on approximations;

therefore, the slight deviations from the accurate enthalpy data in Table 2.2 AgCl

crystallizes in the rock-salt structure and CsI in the cubic body-centered CsCl structure.

2.1.6 Determination of Lattice Energies and Enthalpies

Unfortunately, lattice energies and enthalpies cannot be measured directly but areobtained by indirect means If certain thermodynamic data are known, the latticeenthalpy can be calculated from a BORN–HABERcycle14as shown in Figure 2.4 Suchthermochemical cycles are based on HESS’s law,15which states that the energy dif-ference between two well defined states 1 and 2 is independent of the route fromstate 1 to state 2

The single enthalpies of the BORN–HABERcycle for NaCl are as follows:

ΔlatH° Lattice enthalpy Na+Cl–(−786 kJ mol–1)

ΔfH° Standard enthalpy of formation of crystalline NaCl from the elements(−411 kJ mol–1)

ΔsH° Standard enthalpy of sublimation of sodium metal (+107 kJ mol–1) or ofmolecular NaCl (+196 kJ mol–1).16

1 2 1

14 Published in 1919 by MAX BORN (German physicist; 1882 –1970) and F RITZ HABER (German ist; 1868 –1934).

chem-15 Proposed in 1840 by GERMAIN HENRI HESS (Swiss-Russian chemist; 1802 –1850).

16 For hypothetical molecular NaCl, the enthalpy of sublimation ( Δ sub H°) has been estimated as

196 kJ mol–1.