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Unformatted text preview: Module 4 Unit 2 Inferring Charge Distribution John Pollard University of Arizona In our quest to understand why the particles that make up substances behave differently, we have zoomed in to evaluate the composition, connectivity and geometric locations of atoms. In order to complete our understanding we must now look at how these factors affect the distribution of charge in molecules. What we will find is that the charge distribution is an essential feature that directly relates to the properties of molecular compounds. Explaining Attractions Simple experiments can be done to demonstrate that the distribution of charge is different in the molecules that make up varying substances. For example, a static charge can be built up on a balloon (by rubbing it on your hair) and can in turn be used to manipulate the pathway of a stream of water. This experiment does not work on a hexane. To explain this behavior we first assume that the water molecules have a buildup of charge on one side as compared to the other. This imparts a partial negative and partial positive charge to opposing sides of each molecule which causes the molecules to align with any external electric field (i.e. the charged balloon surface). Therefore, there must not be an asymmetrical charge distribution in hexane molecules. Why? To answer this we need to look closely at how electrons are attracted to atoms involved in a covalent bond. Bond Polarity Uneven charge distribution in molecules arises from covalently bonded atoms having different levels of attraction for shared electrons. For example, consider HCl. The electrons in the H-Cl bond spend more time closer to the Cl atom than the H atom. We therefore say that Cl is more electronegative than H which results in a bond where the charge is asymmetrically distributed (a polar bond)....
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- Fall '08