Chemistry
Mechanisms of Chemical Bonding
Mechanisms of chemical bonding refer to the ways in which atoms are held together in a compound. This includes ionic bonding, where electrons are transferred between atoms, and covalent bonding, where electrons are shared. Additionally, metallic bonding involves a sea of delocalized electrons surrounding positively charged metal ions. Understanding these mechanisms is crucial for comprehending the properties and behavior of different substances.
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7 Key excerpts on "Mechanisms of Chemical Bonding"
eBook - PDFThe Nature of the Mechanical Bond
From Molecules to Machines
- Carson J. Bruns, J. Fraser Stoddart(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
Part 1 Introducing Mechanical Bonds 3 Chapter 1 An Introduction to the Mechanical Bond Conspectus Mechanical bonds are omnipresent and all pervasive. They occupy every nook and cranny of human experience in both static and dynamic settings that range from being close to infinitesimally small to those which can be described as large by comparison. In the domain of the molecular world, the mechanical bond emerges as an additional expression of the nature of the chemical bond in all its manifestations. This emergent phenomenon takes place in the lower reaches of the nanometer length scale as a special architectural feature. It emerges just as soon as it becomes spatially possible to entangle the component parts of molecules, the sizes of which are governed in part by the subnanometer distances between their constituent atoms. The production of chemical compounds composed of mechanically interlocked mole- cules (MIMs) by acts of templation, be they passive or active in their origins, are hand-me- downs from the science of (supramolecular) chemistry beyond the molecule, which affords molecular recognition free rein to exercise its special powers of organization in marshaling the component parts of the MIMs prior to their being transported back into the molecular world by the formation of chemical bonds. This relatively recent extension of bonding in molecules opens the door on a yet little explored field of chemistry—namely that of chemical topology, which leads to the concept of topological isomerism, where two or more molec- ular ensembles may contain the same atoms and chemical bonds, yet cannot be intercon- verted by any deformations that do not involve the breaking and making of chemical bonds. Intersecting the fields of supramolecular chemistry and chemical topology is the discipline of mechanostereochemistry.
eBook - PDFChemistry
An Atoms First Approach
- Steven Zumdahl, Susan Zumdahl, Donald J. DeCoste, , Steven Zumdahl, Steven Zumdahl, Susan Zumdahl, Donald J. DeCoste(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
This concept was reviewed in Section R.10. What is a chemical bond? Chemical bonds can be viewed as forces that cause a group of atoms to behave as a unit. Why do chemical bonds occur? There is no principle of nature that states that bonds are favored or disfavored. Bonds are neither inherently “good” nor inherently “bad” as far as nature is concerned; bonds result from the tendency of a system to seek its lowest possible energy. From a simplistic point of view, bonds occur when collections of atoms are more stable (lower in energy) than the separate atoms. For example, ap- proximately 1652 kJ of energy is required to break a mole of methane (CH 4 ) molecules into separate C and H atoms. Or, from the opposite view, 1652 kJ of energy is released when 1 mole of methane is formed from 1 mole of gaseous C atoms and 4 moles of gaseous H atoms. Thus we can say that 1 mole of CH 4 molecules in the gas phase is 1652 kJ lower in energy than 1 mole of carbon atoms plus 4 moles of hydrogen atoms. Methane is therefore a stable molecule relative to its separated atoms. We find it useful to interpret molecular stability in terms of a model called a chemi- cal bond. To understand why this model was invented, let’s continue with methane, which consists of four hydrogen atoms arranged at the corners of a tetrahedron around a carbon atom. H H C H H A tetrahedron has four equilateral triangular faces. Percent ionic character 0 0 Electronegativity difference 1 2 3 25 50 75 100 LiF KF CsF CsCl KCl NaCl KBr LiCl LiBr Kl CsI HF LiI HCl HBr ICl HI IBr FIGURE 3.7 The relationship between the ionic character of a covalent bond and the electronegativity difference of the bonded atoms. Note that the compounds in red with ionic character greater than 50% are normally considered to be ionic compounds. 111 3.5 The Covalent Chemical Bond: A Model Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.
- David Van Vranken, Gregory A. Weiss(Authors)
- 2018(Publication Date)
- Garland Science(Publisher)
LEARNING OBJECTIVES • Draw arrow-pushing mechanisms to depict chemically reasonable reaction mechanisms. • Identify frontier orbitals involved in bond forming and breaking mechanistic steps. • Describe the chemical basis, strengths, and geometries of hydrogen bonds and proton transfers. • Provide and discuss equations approximating noncovalent interactions. • Discuss and quantify the wide range of noncovalent interactions critical to chemical biology. • Understand how entropy affects systems that can exist in a multitude of atomic arrangements. ChaPter 2 27 The Chemical Origins of Biology 2 I n his book What is Life?, the physicist Erwin Schrödinger (Figure 2.1) argued that all living things are governed by the same physical laws as those encountered in every- day life, such as the laws of thermodynamics and Newton’s laws of motion. His book appeared in the 1940s and influenced a generation of physicists to explore the physi- cal and molecular mechanisms responsible for life. At the time, the molecules at the heart of life were essentially invisible, because spectroscopy and crystallography were not developed for another decade. No exceptions to Schrödinger’s argument have ever been found. Life with all of its surprises, diversity, and complexity seems entirely bounded by the rules of chemistry and physics. Diversity is a fundamental requirement for Darwinian evolution. At the molecular level, oligomerization reactions provide a simple mechanism for generating diverse, structurally related molecules. These reactions can be understood using mechanis- tic arrow-pushing, which is based on quantum mechanical principles. Using arrow- pushing, we will show that simple reactions, applied combinatorially, can explain the formation of all the major classes of biological molecules and the molecular diversity necessary for the emergence of life.
eBook - PDF- Patrick M. Woodward, Pavel Karen, John S. O. Evans, Thomas Vogt(Authors)
- 2021(Publication Date)
- Cambridge University Press(Publisher)
5 Chemical Bonding Changes in crystal structure invariably lead to changes in physical and/or chemical proper- ties. In some cases, these changes can be dramatic, as illustrated by the contrasting properties of the allotropes of carbon (diamond, graphite, graphene, C 60 , etc.); in other cases they are subtle but nonetheless important. To understand the relationship between structure and properties, one must first understand chemical bonding. We begin this chapter with an overview of ionic bonding. From there we move on to the properties of atomic orbitals (AOs) and their interactions to form covalent bonds through the framework of molecular orbital theory. In Chapter 6, we then build upon these principles to describe the formation of bands in extended solids. In this way, covalent and metallic bonding can be understood through a common approach. 5.1 Ionic Bonding Although there are no compounds where the bonding can be described as purely ionic, the ionic model is a useful approximation for many compounds. We begin our treatment of bonding with a brief overview of the factors that determine the strength of ionic bonding in crystalline solids. 5.1.1 Coulombic Potential Energy The coulombic potential energy, U C , between two ions of charge numbers z 1 and z 2 separated by a distance d is: U C ¼ ðz 1 eÞ ðz 2 eÞ 4πε 0 d (5.1) 154 where e is the elementary charge and ε 0 is the electric constant. 1 To estimate the strength of ionic bonding in a crystal, we treat the ions as point charges and use Equation (5.1) to capture all electrostatic interactions in the crystal, both attractive and repulsive. To illustrate, consider the electrostatic interactions in the NaCl structure shown in Figure 5.1. We begin with the Cl − ion in the center of the unit cell and consider the interaction between this ion and all other ions in the crystal.
eBook - PDF- Brian W. Pfennig(Author)
- 2021(Publication Date)
- Wiley(Publisher)
4 An Introduction to Chemical Bonding “A bond does not really exist at all—it is a most convenient fiction.” —Charles Coulson 4.1 THE DEFINITION OF A CHEMICAL BOND Almost every chemical reaction involves the making and/or breaking of a chemical bond. Despite its central importance in the lexicon of chemistry, there continues to be a healthy debate about what a bond actually is, so much so that the British theoretician, Charles Coulson, once quipped that “a bond does not really exist at all—it is a most convenient fiction.” Suppose that I asked my students for their definition of a chemical bond. I imagine that many of them might turn to the fountain of all knowledge known as Wikipedia for a definition. Going straight to the source myself, Wikipedia defines a chemical bond as “a lasting attraction between atoms that enables the formation of a chemical compound,” a circular argument if ever there was one. The Collins English dic- tionary has only a slightly better answer, defining a bond as “a mutual attraction between two atoms resulting from a redistribution of their outer electrons.” Dissatisfied with either of these definitions, I thought I would turn to the OG himself—Gilbert Newton Lewis, who is arguably the godfather of the chemical bond.
eBook - PDFChemical Modelling
Applications and Theory Volume 8
- Michael Springborg(Author)
- 2011(Publication Date)
- Royal Society of Chemistry(Publisher)
The main con-cern when addressing these issues is that they cannot be considered inde-pendent of the mechanically driven reaction and their influence is also dependent on the applied force. The central principal of mechan-ochemistry is that reactivities can be changed when a direction force is applied, and this also extends to modified interactions with the solvent. Changes in steric interactions between molecule and solvent can change the separation of the species and affect polarization of the components of the system. This also applies to impurities present in the solution. Varia-tions in temperature can alter the rupture pathway as at elevated tem-peratures thermal effects play a more significant role in determining the outcome of the experiment. By comparing 0 K calculations with those at elevated temperatures, first principles calculations can be used to pinpoint which processes are unique to mechanical activation. The configuration of the substrate/molecule binding as the precise atomic arrangement can affect the strength of the attachment and how likely this is to rupture compared to a bond within the molecule. This is can be dependent on how the surface atom involved in the attachment interaction is bonded to other atoms in the substrate or tip. The nature of the bond can also play a role: if there is a covalent interaction which is known to be weaker than other bonds in the system this will most likely break first but again is subject to considerations of its position in the system. The inclusion of a weak che-mical bond in a system lies behind the concept of a mechanophore, in which mechanical energy is funneled through the molecule to the weakest interaction which triggers the mechanochemical reaction. These considerations provide an overview of the complexity of mechan-ochemistry processes and approaches to their interpretation by applying first principles techniques to model systems.
eBook - PDF- Dunod Editeur, M S A Editeur, J R Lalanne(Authors)
- 1996(Publication Date)
- World Scientific(Publisher)
PART m TWO COMPLEMENTARY DESCRIPTIONS OF CHEMICAL BONDING This page is intentionally left blank CHAPTER m Mechanical aspects of chemical bonding I. Basics I. The Born-Oppenheimer approximation and the potential energy of nuclei The theoretical study of a molecule requires the use of quantum mechanics. It: various stationary states are described as solutions of the eigenvalue equation o the hamiltonian operator H H x ¥ = E v f (ni.I.l) This operator is equal to the sum of the kinetic energy operator T and the potential energy operator V H =T+V (III.I.2) The operator V corresponds to the multiplication by the potential energy function. [The definition of this function and the form of the potential energy function used in this chapter are given in Appendix 6 (Sec. III.II.4)]. The wave functions *P are functions of 4n variables (three space variables and one spin variable for each of the n particles). Thus, the theoretical treatment of a such a molecule is always a complex and difficult task. The aim of this chapter is to show — When a chemical bond can be considered, from a mechanical point of view, as equivalent to a kind of spring binding the two atoms. — That the acting forces can then be attributed to the density of electronic charges at each point in space and to charges on the various nuclei. 166 Mechanical aspects In order to appreciably reduce the complexity of the equations, we shall begin with the example of the hydrogen molecule. We shall then extend the results to other molecules. The Hamiltonian operator and wave function of the hydrogen molecule The hydrogen molecule consists of two protons A and B of charge q (1.60 x 1(H 9 C) and mass m p (1.67 x 10~ 21 kg) and two electrons, 1 and 2, with charge - q and mass m e (9.11 x l O 3 1 kg) (Fig.III.I.l). Figure III.1.1: Distances between electrons and nuclei in the hydrogen molecule. Forces between these charged particles are attractive and repulsive coulombic forces.
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