Types of Chemical Bonds

Types of Atoms Type of Bond Characteristic of Bond 

In Chapter 8 we discussed that metals tend to have low ionization energies (their electrons are relatively easy to ronove) and that nonmetals tend to have negative electron affinities (they readily gain electrons). When a metal bonds with a nonmetal, it transfers one or more electrons to the nonmetal. The metal atom becomes a cation and the nonmetal atom an anion. These oppositely chaiged ions then attract one another, lowering their overall potential energy as described by Coulomb s law. The resulting bond is an ionic bond. 

We also discussed in Chapter 8 that nonmetals tend to have high ionization energies (their electrons are relatively difficult to ranove). Therefore when a nonmetal bonds with another nonmetal, neither atom transfers electrons to the other. Instead, the two atoms 

While the exact lecatiea ef let i wet critical, ia this heek we will first place Rets siagly hefere pairiag (except far heliaM, which always has twe pairefi fiats sigaifyiag its fiuet). 

Each dot represents a valence electron. The dots are placed around the element s symbol with a maximum of two dots per side. The Lewis symbols for all of the period 2 elanents are drawn in a similar way  

Diamond, composed of carbon atoms bonded together to produce one of the hardest materials known, makes a beautiful gemstone. 

The manner in which atoms are bound together has a profound effect on the chemical and physical properties of substances. For example, both graphite and diamond are composed solely of carbon atoms. However, graphite is a soft, slippery material used as a lubricant in locks, and diamond is one of the hardest materials known, valuable both as a gemstone and in industrial cutting tools. Why do these materials, both composed solely of carbon atoms, have such different properties The answer lies in the different ways in which the carbon atoms are bound to each other in these substances. 

To understand the behavior of natural materials, we must understand the nature of chemical bonding and the factors that control the structures of compounds. In this chapter, we will present various classes of compounds that illustrate the different types of bonds. We will then develop models to describe the structure and bonding that characterize the materials found in nature. 

I AIMS To learn about ionic and covalent bonds and explain how they are formed. To learn about the polar covalent bond. 

Atoms can interact with one another in several ways to form aggregates. We will consider specific examples to illustrate the various types of chemical bonds. 

I he world around us is composed almost entirely of compounds and JL. mixtures of compounds Rocks, coal, soil, petroleum, trees, and human bodies are all complex mixtures of chemical compounds in which different kinds of atoms are bound together. Substances composed of unbound atoms do exist in nature, but they are very rare. Examples are the argon in the atmosphere and the helium mixed with natural gas reserves. 

Silicon and carbon are next to each other in Group 4A on the periodic table. From our knowledge of periodic trends, we might expect SiOi and CO2 to be very similar. But SiOi is the empirical formula of silica, which is found in sand and quartz, whereas carbon dioxide is a gas, a product of respiration. Why are they so different We will be able to answer this question after we have developed models for bonding. 

What is a chemical bond There is no simple and yet complete answer to this question. In Chapter 2 we defined bonds as forces that hold groups of atoms together and make the atoms function as a unit. 

As soon as the forces of distribution (Chapter 3) have brought a drug molecule close to a receptor, the two are sure to be hurled into fleeting contact by thermal agitation. How durable and how selective that contact may be depends on the chemical nature of both drug and receptor, and on the type of bonds they can form. 

In what follows, the various types of chemical bond will be discussed with reference to the part that each can play in drug-receptor unions. This outline will lead to a discussion of adsorption, which is much the same kind of phenomenon but described in different terms. The chapter will conclude with examples of highly selective binding found in non-biological settings, serving to remind us that selectivity flourishes independently of life. 

In the past, it used to be claimed that some drugs acted by physical and some by chemical means. It was also suggested that the physical properties of a drug were responsible for getting it to the site of action, but there it reacted chemically. In these connotations, the word chemical was strangely limited to the formation of covalent bonds. 

Such a contrast between physical and chemical now seems unreal. Langmuir (1916, 1917) made it clear that physics and chemistry are interwoven every substance has physical properties, and the physical properties which it has are the inevitable result of its particular chemical structure. Thus 

However, it must not be supposed that difficulty encountered in washing a drug out of a tissue necessarily implies covalent bond formation. The drug molecule may simply be clathrate, i.e. physically imprisoned in the folds of a macromolecule (biopolymer). The textile industry has long made use of this principle in dyeing cotton fabrics with long, narrow molecules of the type of Sky Blue 8,8). These are polyazo-dyes of low molecular weight, known as direct 

The formation of a bond between two hydrogen atoms, (a) Two separate hydrogen atoms, (b) When two hydrogen atoms come close together, the two electrons are attracted simultaneously by both nuclei. This produces the bond. Note the relatively large electron probability between the nuclei, indicating sharing of the electrons. 

A compound that results when a metal reacts with a nonmetal to form cations and anions 

A type of bonding in which atoms share electrons 

A covalent bond in which the electrons are not shared equally because one atom attracts the shared electrons more than the other atom 

All of these early studies, however, contained, in addition to suggestions that have since been incorporated into the present theory, many others that have been discarded. The refinement of the electronic theory of valence into its present form has been due almost entirely to the development of the theory of quantum mechanics, which has not only provided a method for the calculation of the properties of simple molecules, leading to the complete elucidation of the phenomena involved in the formation of a covalent bond between two atoms and dispersing the veil of mystery that had shrouded the bond during the decades since its existence was first assumed, but has also introduced into chemical theory a new concept, that of resonance, which, if not entirely unanticipated in its applications to chemistry, nevertheless had not before been clearly recognized and understood. 

In the following sections of this chapter there are given, after an introductory survey of the types of chemical bonds, discussions of the concept of resonance and of the nature of the one-electron bond and the electron-pair bond. 

It is convenient to consider three general extreme types of chemical bonds electrostatic bonds, covalent bonds, and metallic bonds. This classification is not a rigorous one for, although the bonds of each extreme 

8 This was treated independently at about the same time by W. Kossel, Ann. Physik 49, 229 (1916). 

The energy of interaction between a pair of ions can be calculated using Coulomb s law in the form 

For example, in solid sodium chloride the distance between the centers of the Na and cr ions is 2.76 A (0.276 nm), and the ionic energy per pair of ions is 

The negative sign indicates an attractive force. That is, the ion pair has lower energy than the separated ions. For a mole of pairs of Na+ and Cl- ions, the energy of interaction is 

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What is a chemical bond Although there are several possible ways to answer this question, we will define a bond as a force that holds groups of two or more atoms together and makes them function as a unit. For example, in water the fundamental unit is the H—O—H molecule, which we describe as 

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Atoms combine with one another to give compounds having properties different from the atoms they contain The attractive force between atoms m a compound is a chemical bond One type of chemical bond called an ionic bond, is the force of attraction between oppositely charged species (ions) (Figure 1 4) Ions that are positively charged are referred to as cations, those that are negatively charged are anions... 

Chemical bonds are strong forces of attraction which hold atoms together in a molecule. There are two main types of chemical bonds, viz. covalent and ionic bonds. In both cases there is a shift in the distribution of electrons such that the atoms in the molecule adopt the electronic configuration of inert gases. 

Carbon has six electrons around the atomic core as shown in Fig. 2. Among them two electrons are in the K-shell being the closest position from the centre of atom, and the residual four electrons in the L-shell. TTie former is the Is state and the latter are divided into two states, 2s and 2p. The chemical bonding between neighbouring carbon atoms is undertaken by the L-shell electrons. Three types of chemical bonds in carbon are single bond contributed from one 2s electron and three 2p electrons to be cited as sp bonding, double bond as sp and triple bond as sp from the hybridised atomic-orbital model. 

Two types of chemical bonds, ionic and covalent, are found in chemical compounds. An ionic bond results from the transfer of valence electrons from the atom of an electropositive element (M) to the atom(s) of an electronegative element (X). It is due to coulombic (electrostatic) attraction between the oppositely charged ions, M (cation) and X (anion). Such ionic bonds are typical of the stable salts formed by combination of the metallic elements (Na, K, Li, Mg, etc.) with the nonmetallic elements (F, Cl, Br, etc.). As an example, the formation of the magnesium chloride molecule from its elemental atoms is shown by the following sequence ... 

Second, the theory of hybrid bond orbitals was utilized recently to discover a new type of chemical bond involving the resonating unshared electron pair.30 31 For example, bis(bistrimethylsilylmethyl) tin(II), (CH3)3 Si 2HCSnCH Si(CH3)3 2, forms dimers in the solid state having a tin-tin bond characterized by resonance of an unshared electron pair or... 

In chemistry, perhaps because of the significance in visualizing molecular strac-ture, there has been a focus on how students perceive three-dimensional objects from a two-dimensional representation and how students mentally manipulate rotated, reflected and inverted objects (Stieff, 2007 Tuckey Selvaratnam, 1993). Although these visualization skills are very important in chemistry, it is evident that they are not the only ones needed in school chemistry (Mathewson, 1999). For example, conceptual understanding of nature of different types of chemical bonding, atomic theory in terms of the Democritus particle model and the Bohr model, and... 

Direct detection and investigation of the intermediates are of great importance, not only for the solution of mechanistic tasks, but also for studies of their structure. As a rule these intermediates have unusual structures, open electronic shells, delocalized unpaired electrons and new types of chemical bonds. That is why their investigation sets new problems for the general theory of chemical structure. 

Atoms in a molecule are joined by bonds. Bonds are formed when the valence or outermost electrons of two or more atoms interact. The nature of the bond between atoms goes a long way toward determining the properties of the molecule. Chapter 5 introduced the two common types of chemical bonds covalent and ionic. Elements with similar electronegativities share electrons and form covalent bonds. But elements with greatly different electronegativities exchange one or more electrons. This is called an ionic bond. 

Although the band model explains well various electronic properties of metal oxides, there are also systems where it fails, presumably because of neglecting electronic correlations within the solid. Therefore, J. B. Good-enough presented alternative criteria derived from the crystal structure, symmetry of orbitals and type of chemical bonding between metal and oxygen. This semiempirical model elucidates and predicts electrical properties of simple oxides and also of more complicated oxidic materials, such as bronzes, spinels, perowskites, etc. 

However, It has been found that in many cases, simple models of the properties of atomic aggregates (monocrystals, poly crystals, and glasses) can account quantitatively for hardnesses. These models need not contain disposable parameters, but they must be tailored to take into account particular types of chemical bonding. That is, metals differ from covalent crystals which differ from ionic crystals which differ from molecular crystals, including polymers. Elaborate numerical computations are not necessary. 

There are at least four types of chemical bonding. Some crystals have open atomic structures, while others are close-packed. Also, many crystals are anisotropic. Therefore, although making hardness measurements is relatively simple, understanding the measured values is not simple at all. 

For interpreting indentation behavior, a useful parameter is the ratio of the hardness number, H to the shear modulus. For cubic crystals the latter is the elastic constant, C44. This ratio was used by Gilman (1973) and was used more generally by Chin (1975) who showed that it varies systematically with the type of chemical bonding in crystals. It has become known as the Chin-Gilman parameter (H/C44). Some average values for the three main classes of cubic crystals are given in Table 2.1. 

The difference of the Chin-Gilman parameter for differing types of chemical bonding accounts for the Tabor constant not being three for non-metals. 

Physical hardness can be defined to be proportional, and sometimes equal, to the chemical hardness (Parr and Yang, 1989). The relationship between the two types of hardness depends on the type of chemical bonding. For simple metals, where the bonding is nonlocal, the bulk modulus is proportional to the chemical hardness density. The same is true for non-local ionic bonding. However, for covalent crystals, where the bonding is local, the bulk moduli may be less appropriate measures of stability than the octahedral shear moduli. In this case, it is also found that the indentation hardness—and therefore the Mohs scratch hardness—are monotonic functions of the chemical hardness density. 

The principal intention of the present book is to connect mechanical hardness numbers with the physics of chemical bonds in simple, but definite (quantitative) ways. This has not been done very effectively in the past because the atomic processes involved had not been fully identified. In some cases, where the atomic structures are complex, this is still true, but the author believes that the simpler prototype cases are now understood. However, the mechanisms change from one type of chemical bonding to another. Therefore, metals, covalent crystals, ionic crystals, and molecular crystals must be considered separately. There is no universal chemical mechanism that determines mechanical hardness. 

As noted in Chapter 2, sand, silt, clay, and organic matter do not act independently of each other in soil. Thus, one or several types of chemical bonds or interactions—ionic, polar covalent, covalent, hydrogen, polar-polar interactions, and van der Waals interactions—will be important in holding soil components together. The whole area of chemical bonding is extremely complex, and thus, in addition to specific bonding considerations, there are also more... 

Packed columns are still used extensively, especially in routine analysis. They are essential when sample components have high partition coefficients and/or high concentrations. Capillary columns provide a high number of theoretical plates, hence a very high resolution, but they cannot be used in all applications because there are not many types of chemically bonded capillary columns. Combined use of packed columns of different polarities often provides better separation than with a capillary column. It sometimes happens that a capillary column is used as a supplement in the packed-column gas chromatography. It is best, therefore, to house the capillary and packed columns in the same column oven and use them selectively. In the screening of some types of samples, the packed column is used routinely and the capillary column is used when more detailed information is required. 

In Sections 9-3 and 9-4, we will show you two types of chemical bonds ionic and covalent. It is important to be able to represent compounds in terms of the atoms and valence electrons that make up the chemical species (compounds or polyatomic ions). One of the best ways is to use Lewis symbols and structures. 

Based on equations (2-5) with initial data calculated with quantum-mechanical techniques [6-8], the values of P0-parameters of the majority of elements being tabulated constant values for each valence atom orbital were calculated. Mainly covalent radii were applied as a dimensional characteristic for calculating PE-parameter - by main type of chemical bond of interactions considered (table 1). For hydrogen atom also the value of Bohr radius and value of atomic ( metal ) radius were applied. 

We know that the three types of chemical bonds that exist between atoms are non polar covalent bonds, polar covalent bonds and ionic bonds. We are already familiar with the idea that it is helpful to think of these as making up a bonding continuum. Non polar covalent bonding lies at one end of the continuum and ionic bonding at the other polar covalent bonding lies between these two extremes. 

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