Chapter 10 - Bonding - CHEMICAL BONDING We have seen how it...

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81 CHEMICAL BONDING We have seen how it is most favorable for atoms to have a full outer shell (valence shell) of electrons. This is the driving force for atomic reactivity . Atoms seek to attain this electron configuration - either by losing, gaining or sharing outer shell electrons. The number of outer shell electrons accounts for the observed valence of atoms. Atomic Lewis Symbols: Outer Shell Electron Configuration Observed valence Atomic Lewis Symbol 2s 2p 1 Li 2 Be 3 B 4 C 3 : N 2 : O 1 : F 0 : Ne : excited state configurations allow greater valence & better achieve (on bonding) a full (or almost full valence shell) . . .. .. .. .. .. .
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82 Ionic Bonding Atoms to right of the periodic table ( metals ) have few valence electrons, whereas atoms to the left ( nonmetals ) have nearly filled valence shells. It is favorable for the metal to lose its valence electrons to the non-metal to form ions with filled outer shells - octets These ions arrange themselves alternately in a crystal lattice to maximize the electrostatic attraction between the ions of opposite charge, while minimizing the repulsion between ions of the same charge.
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83 The cost of forming ions from atoms is the ionization energy of the metal atom (or, eg., the sum of the 1 st & 2 nd ionization energies in the case of forming 2+ ions) minus the electron affinity of the nonmetal atom (or sum of successive electron affinities for multiply negatively charged anions). Low ionization energy of metal + high electron affinity of non-metal makes ionic bond formation favorable. There are other contributing terms (i.e. contributing to H ° f of ionic solid ) ... we must break bonds of elemental materials to form ionic compound from elements having produced gaseous ions, there is a "lattice energy", H ° lattice , associated with the electrostatic forces between ions - attractive forces emphasized over repulsive
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84 Lattice Energy The lattice energy... increases with increasing charge on ions decreases with increasing distance between ions (sum of ionic radii ) E = k q + q - /r Of course, the lattice energy is more complex involving a sum over all pairs of ions in the crystal. But, the electrostatic attraction between neighboring ions of opposite charge gives the right trends. eg. MgCl 2 2527 kJ/mol Na 2 O 2570 kJ/mol MgO 3890 kJ/mol
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85 Covalent Bonding Bonding between non-metal atoms requires both atoms attempting to complete their octets . This is possible if the atoms share some of their valence electrons - the covalent bond ... eg. : F + F : : F : F : : F F : .. .. .. .. .. .. lone pair of electrons covalent bond pair of electrons .. .. .. .. .. ..
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86 egs. of covalent bonds ... Ammonia: NH 3 N H 1s 2 2s 2 2p 3 1s 1 H H : N H : N H H H Carbon dioxide: CO 2 C O 1s 2 2s 2 2p 2 1s 1 2s 2 2p 4 O C O O=C=O Nitrogen: : N N : 5 1 .
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