Unit 3- Module 3 - Module 3 Unit 3 Characterizing Ionic...

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Module 3 Unit 3 Characterizing Ionic Networks John Pollard University of Arizona Sodium chloride is an amazing substance if you think about it. It can be made by reacting chlorine gas, an extremely reactive and toxic murky yellow/green gas with a dubious history with sodium, a metal that reacts violently in the presence of water. When combined, the product is a white stable crystalline material that you can put on food! What is it that changes in both molecular chlorine and sodium metal that causes such an extreme transformation? For some visuals, check this video out about aluminum chloride. http://www.youtube.com/periodicvideos#p/u/1/BXCfBl4rmh0 To start, let’s first look at what differentiates ionic substances from molecular compounds. Ionic compounds result from result from of metals and nonmetals and are typically solids at room temperature with many having very high melting points. How can we explain and predict the physical properties of ionic compounds? A close look at the interactions on the submicroscopic level provides an answer. Non-metal atoms have higher ionization energies and electronegativies than those of metals which results in nonmetals have a much higher affinity for electrons than metals. As an example, let’s consider F 2 , HF and NaF. In F 2 the covalent bond is non-polar and the charge is equally distributed between the fluorine atoms. In HF, the bond is extremely covalent with the electrons spending a significant amount of time around the fluorine due to the large electronegativity difference between H and F. This increased polarity introduces more ionic character to the interaction meaning that the ¯ and δ δ + charges are comings closer to being full – and + charges. In the case of sodium interacting with fluorine, the electronegativity difference
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is so great that both achieve an octet by the transfer of an electron from sodium to fluorine. Thus, -1 and + 1 ions are formed with the nonmetal (F) becoming the anion(-) and the metal (Na) becoming the cation(+ ). When the electronegativity differences become this great (in general greater than 1.7…a completely arbitrary number) we classify the interaction as ionic. When ions are formed, the attractive forces they feel for other ions of opposing charge is not directional
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This note was uploaded on 12/08/2009 for the course CHEM 151 taught by Professor Staff during the Fall '08 term at University of Arizona- Tucson.

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Unit 3- Module 3 - Module 3 Unit 3 Characterizing Ionic...

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