Chapter 8 Lecture Notes - Chapter 8 Concepts of Chemical...

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Chemical Bonding Chapter 8 Concepts of Chemical Bonding
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Chemical Bonding Chemical Bonds Three types: – Ionic Electrostatic attraction between ions – Covalent Sharing of electrons – Metallic Metal atoms bonded to several other atoms
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Chemical Bonding Lewis symbols A convenient way to keep track of the valence electrons in an atom or molecule Lewis dot symbol Each dot is one valence electron
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Chemical Bonding Lewis structures for 16 elements It is rare to use Lewis pictures for other elements (transition metals, etc.)
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Chemical Bonding Ionic Bonding
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Chemical Bonding Energetics of Ionic Bonding it takes 495 kJ/mol to remove 1 electron from sodium. 2Na(s) + Cl 2 (g) -------> 2NaCl(s)
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Chemical Bonding Energetics of Ionic Bonding We get 349 kJ/mol back by giving 1 electron each to 1 mole of chlorine.
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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!
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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.
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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
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Chemical Bonding Lattice Energy E el = κ Q 1 Q 2 d
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Chemical Bonding Lattice Energy Lattice energy, then, increases with the charge on the ions. It also increases with decreasing size of ions.
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Chemical Bonding Energetics of Ionic Bonding By accounting for all three energies (ionization energy, electron affinity, and lattice energy), we can get a good idea of the energetics involved in such a process. Na(s) + 1/2Cl 2 (g) -----> NaCl(s)
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Chemical Bonding Energetics of Ionic Bonding These phenomena also help explain the “octet rule.” Elements tend to lose or gain electrons once they attain a noble gas configuration because energy would be expended that cannot be overcome by lattice energies.
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Chemical Bonding Covalent Bonding In these bonds atoms share electrons.
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Chemical Bonding Covalent Bonding There are several electrostatic interactions in these bonds: – Attractions between electrons and nuclei – Repulsions between electrons – Repulsions between nuclei
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Chemical Bonding Polar Covalent Bonds Although atoms often form compounds by sharing electrons, the electrons are not always shared equally.
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