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CHS1440Ch08 - Chemical Bonding Chapter 8 Concepts of...

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Unformatted text preview: Chemical Bonding Chapter 8 Concepts of Chemical Bonding Chemical Bonding Chemical Bonds • Three basic types of bonds: ¾ Ionic • Electrostatic attraction between ions ¾ Covalent • Sharing of electrons ¾ Metallic • Metal atoms bonded to several other atoms Chemical Bonding Ionic Bonding Chemical Bonding Energetics of Ionic Bonding As we saw in the last chapter, it takes 495 kJ/mol to remove electrons from sodium. Chemical Bonding Energetics of Ionic Bonding We get 349 kJ/mol back by giving electrons to chlorine. The energy required to remove one electron from Na(g) and add it to Cl (g) is 496 +(-349) = 147 kJ/mol It is endothermic!!! Chemical Bonding Energetics of Ionic Bonding • But these numbers don’t explain why the reaction of sodium metal and chlorine gas to form sodium chloride is so exothermic! Chemical Bonding Energetics of Ionic Bonding • There must be a third piece to the puzzle. • What is as yet unaccounted for is the electrostatic attraction between the newly formed sodium cation and chloride anion. Chemical Bonding Lattice Energy This third piece of the puzzle is the lattice energy: The energy required to completely separate a mole of a solid ionic compound into its gaseous ions. The energy associated with electrostatic interactions is governed by Coulomb’s law: E el = κ Q 1 Q 2 d Chemical Bonding Lattice Energy • Lattice energy, then, increases with the charge on the ions. • It also increases with decreasing size of ions. Chemical Bonding Energetics of Ionic Bonding By accounting for all four energies (evaporation of sodium, ionization energy, electron affinity, and lattice energy), we can get a good idea of the energetics involved in such a process. ∆ H = -411kJ/mol Chemical Bonding Energetics of Ionic Bonding • These phenomena also helps explain the “octet rule.” Atoms tend to gain, lose or share electrons until they are surrounded by eight valence electrons. • Metals, for instance, tend to stop losing electrons once they attain a noble gas configuration because energy would be expended that cannot be overcome by lattice energies. Chemical Bonding Covalent Bonding • In these bonds atoms share electrons. • There are several electrostatic interactions in these bonds: ¾ Attractions between electrons and nuclei ¾ Repulsions between electrons ¾ Repulsions between nuclei Chemical Bonding Polar Covalent Bonds • Although atoms often form compounds by sharing electrons, the electrons are not always shared equally. • Fluorine pulls harder on the electrons it shares with hydrogen than hydrogen does. • Therefore, the fluorine end of the molecule has more electron density than the hydrogen end. Chemical Bonding Electronegativity: • The ability of atoms in a molecule to attract electrons to itself....
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CHS1440Ch08 - Chemical Bonding Chapter 8 Concepts of...

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