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Chapter 9 chemical bonds 503 it may be more

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Chapter 9 Chemical Bonds503
it may be more appropriate to draw the covalent bond aswith the electrons in the bond being nearer to the F atom than the H atom. Becausethe electrons in the bond are nearer to the F atom, this side of the molecule takeson a partial negative charge, which is represented byδ− (δis the lowercase Greekletter delta). The other side of the molecule, the H atom, adopts a partial positivecharge, which is represented byδ+:A covalent bond between different atoms that attract the shared electrons bydifferent amounts and cause an imbalance of electron distribution is called apolarcovalent bond14.Technically, any covalent bond between two different elements is polar. However,the degree of polarity is important. A covalent bond between two differentelements may be so slightly imbalanced that the bond is, essentially, nonpolar. Abond may be so polar that an electron actually transfers from one atom to another,forming a true ionic bond. How do we judge the degree of polarity?Scientists have devised a scale calledelectronegativity15, a scale for judging howmuch atoms of any element attract electrons. Electronegativity is a unitlessnumber; the higher the number, the more an atom attracts electrons. A commonscale for electronegativity is shown inFigure 9.2 "Electronegativities of theElements".14. The unequal sharing ofelectrons in a covalent bond.15. A qualitative scale for judginghow much atoms of anyelement attract electrons.Chapter 9 Chemical Bonds9.4 Other Aspects of Covalent Bonds504
Figure 9.2Electronegativities of the ElementsElectronegativities are used to determine the polarity of covalent bonds.The polarity of a covalent bond can be judged by determining thedifferenceof theelectronegativities of the two atoms involved in the covalent bond, as summarizedin the following table:Electronegativity DifferenceBond Type0nonpolar covalent0–0.4slightly polar covalent0.4–1.9definitely polar covalent>1.9likely ionicChapter 9 Chemical Bonds9.4 Other Aspects of Covalent Bonds505
EXAMPLE 7What is the polarity of each bond?1.C–H2.O–HSolutionUsingFigure 9.2 "Electronegativities of the Elements", we can calculate thedifference of the electronegativities of the atoms involved in the bond.1.For the C–H bond, the difference in the electronegativities is 2.5 − 2.1 =0.4. Thus we predict that this bond will be slightly polar covalent.2.For the O–H bond, the difference in electronegativities is 3.5 − 2.1 = 1.4,so we predict that this bond will be definitely polar covalent.Test YourselfWhat is the polarity of each bond?1.Rb–F2.P–ClAnswers1.likely ionic2.polar covalentThe polarity of a covalent bond can have significant influence on the properties ofthe substance. If the overall molecule is polar, the substance may have a highermelting point and boiling point than expected; also, it may or may not be soluble invarious other substances, such as water or hexane.

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