Representing covalent bonds

Comparing co(/atent bonds (Pith other bonds 

The properties of ionic cmd covalent compounds eire different. Table 6-1 shows how the compounds compare. (Note For the cleissification between metals and nonmetals, see Chapter 3.) 

Pmperty Ionic Compounds (Salts) Covalont Compounds 

Bonds occur between A metal and a nonmetal Two nonmetals 

State of the compound at room temperature Usually solid Can be solid, liquid, or gas 

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Organic chemists have devised a number of shortcuts to speed the writing of structural formulas Sometimes we leave out unshared electron pairs but only when we are sure enough m our ability to count electrons to know when they are present and when they re not We ve already mentioned representing covalent bonds by dashes In condensed structural formulas we leave out some many or all of the covalent bonds and use sub scripts to indicate the number of identical groups attached to a particular atom These successive levels of simplification are illustrated as shown for isopropyl alcohol ( rub bmg alcohol )... 

The native conformation of proteins is stabilized by a number of different interactions. Among these, only the disulfide bonds (B) represent covalent bonds. Hydrogen bonds, which can form inside secondary structures, as well as between more distant residues, are involved in all proteins (see p. 6). Many proteins are also stabilized by complex formation with metal ions (see pp. 76, 342, and 378, for example). The hydrophobic effect is particularly important for protein stability. In globular proteins, most hydrophobic amino acid residues are arranged in the interior of the structure in the native conformation, while the polar amino acids are mainly found on the surface (see pp. 28, 76). 

In this chapter we shall examine first a convenient way to represent covalent bonds on paper and then discuss the factors that determine the composition of molecules, the e.xtent of electron sharing, the strength of bonding, and the geometry of molecules. 

Propane, C3Hg, has a structure in which the three carbon atoms are bonded in a row, each end carbon is bonded to three hydrogens, and the middle carbon is bonded to two hydrogens. Draw the structural formula, using lines between atoms to represent covalent bonds. 

LEWIS STRUCTURE OF ETHANE DOTS REPRESENT ELECTRONS AND UNES REPRESENT COVALENT BONDS. 

The bonding parameters (d1, D ) of all covalent bonds, irrespective of order, lie within the field defined by the curves ABE. It is postulated that the field EBF represents covalent bonds under high pressure. The events at point F are open to conjecture. 

The chemical constitution of an EM is represented by a "be" matrix ["bond-electron matrix]. A be-matrix B representing an EM(B) consisting of n atoms is an nXn matrix with integral entries where the off-diagonal entries b- j represent covalent bonds between the atoms A and Aj, and the diagonal entries b- correspond to the numbers of free unshared valence electrons on the atom A . It is easy to see that B is symmetric, that the row/column sums, bi = Eby = bji are the numbers of valence... 

When atoms share three pairs of electrons, they form a triple bond. Diatomic nitrogen contains a triple bond, as you can see in Figure 3.19. Try the following problems to practise representing covalent bonding using Lewis structures. Watch for multiple bonding ... 

Figure 8-30 Schematic Diagram of Bonds Within and Between Polypeptide Chains in Dough. Solid lines represent covalent bonds, dotted lines other bonds. (1) Intramolecular disulfide bond, (2) free sulfhydryl group, (3) intermolecular disulfide bond, (4) ionic bond, (5) van der Waals bond, (6) interpeptide hydrogen bond, (7) side chain hydrogen bond. Source From A.H. Bloksma, Rheology of Wheat Flour Dough, J. Texture Studies, Vol. 3, pp. 3-17,1972.

Figure 4 BeCb structures (a) monomer, (b) dimer, and (c) polymer. Solid lines represent covalent bonds and arrowed lines represent coordinate (Lewis base — Lewis acid) bonds. Note that the depicted structure is in resonance with other formal electron placement schemes. Thus, on average, all Be-Cl interactions appear equivalent...

N.B. The lines here do not represent covalent bonds, but merely help to represent the 3-D structure. 

The Lewis model (see Section 3.8) represents covalent bonds as shared valence-electron pairs positioned between two nuclei, where they presumably are involved in net attractive interactions that pull the nuclei together and contribute to the strengthening of the bond. The mechanism cannot be explained by classical physics, and is examined through quantum mechanics in Chapter 6. 

In other situations, the neutral oxygen atom may share electrons with one or more other atoms, in order to act as though it has a complete valence shell part of the time. These shared electrons represent covalent bonds and result in the formation of molecular compounds, as shown here ... 

A double bond is represented by two pairs of dots, etc. Dots representing nonbonded outer-shell electrons are placed adjacent to the atoms with which they are associated, but not between the atoms. Formal charges (e.g. +, -, 2+, etc.) are attached to atoms to indicate the difference between the positive nuclear charge (atomic number) and the total number of electrons (including those in the inner shells) on the formal basis that bonding electrons are shared equally between atoms they join. (Bonding pairs of electrons are usually denoted by lines, representing covalent bonds, as in line FORMULAe.)... 

Figure 7-1 A representation of the crystal structure of NaCl. Each CC ion (green) is surrounded by six sodium ions, and each Na ion (gray) is surrounded by six chloride ions. Any NaCl crystal includes bilUons of ions in the pattern shown. Adjacent ions actually are in contact with one another in this drawing, the strucmre has been expanded to show the spatial arrangement of ions. The lines do not represent covalent bonds. Compare with Figure 2-7, a space-filhng drawing of the NaCl structure.

In this book, lines between atom symbols represent covalent bonds between those atoms. Nonbonding... 

FIGURE 11.6 Hydrogen bonding in water, ammonia, and hydrogen fluoride. Solid lines represent covalent bonds, and dotted lines represent hydrogen bonds. 

The thicker lines represent covalent bonds. The formula for this ion is OsSi—O—SiOs ", or SiaOv . Each oxygen that is not shared has a negative charge. There are six of these oxygens and therefore the charge on the ion is 6-. 

Some covalent bonds connect two identical atoms of the elements that exist naturally as diatomic molecules (i.e., Hg, Fg, Clg, Brg, l, Og, and Ng). Lewis dot structures of the type discussed earlier can also be used to represent covalent bonding. 

Fig. 1. Six basic combinations of two polymers, represented by solid and dashed lines, respectively, and the dots representing covalent bonds, (a) A polymer blend, with no bonding between the chains (b) a graft copoljmier, with the dashed polymer bonded to the side of the solid line poisoner (c) a block copoljmier, where the chains are bonded end-on-end (d) an AB-cross-linked copoljmier, where one poljmier is bonded to the other, but not to itself (e) an interpenetrating poljmier network of two cross-linked poljmiers (f) a semi-interpenetrating polymer network, where only one of the poljmiers is cross-linked.

Recall from Chapter 5 that when nonmetals bond with other nonmetals, a molecular compoimd results. Molecular compounds contain covalent bonds in which electrons are shared between atoms rather than transferred. In Lewis theory, we represent covalent bonding by allowing neighboring atoms to share some of their valence electrons in order to attain octets (or duets for hydrogen). For example, hydrogen and oxygen have the Lewis structures ... 

In covalent bonds, atoms share their electrons. We represent covalent bonding in Lewis theory by letting atoms share their dots such that some dots count for the octet of more than one atom. For example, we learned in Chapter 3 that elemental chlorine exists as the diatomic (two atom) molecule CI2. Lewis theory explains why. Consider the Lewis structure of chlorine ... 

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