CHM_122a__Lecture_note[1].doc

CHM_122a__Lecture_note[1].doc - CHM 122 GENERAL INORGANIC...

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CHM 122 – GENERAL INORGANIC CHEMISTRY 2UNITS MODULE I: CHEMICAL BONDING. OBJECTIVES a) To study the formation of the major types of chemical bonds usually encountered in compounds. b) To study the general characteristic properties of the compounds formed by the different chemical bonding. The types of chemical bonding to be considered are i) Ionic or electrovalent bonding ii) Covalent bonding iii) Coordinating covalent (dative) bonding iv) Metallic bonding v) Hydrogen bonding c) In this module you will also learn about the following concept or terms: (i) Valence electrons (ii)Ionization energy (iii)Electron affinity (iv)Octet rule and exceptions to the Octet rule (v)Lewis formulas or structures (vi) Non – bonding electrons, bonding pairs of electrons and lone pairs of electrons (vii) Polar compounds (viii)Intermolecular forces of attraction etc 1
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INTRODUCTION i) Valence (or Outermost Shell) Electrons Valence electrons are those involved in forming compounds. They are the outermost shell electrons of atoms. For example, nitrogen has the electronic configuration 1s 2 2s 2 3p 3 . The outermost shell electrons are those in the 2s and 2p orbitals. Atom and Electronic Configuration Valence Electrons N[1s 2 2s 2 2p 3 ] 2s 2 2p 3 = 5 O[1s 2 2s 2 2p 4 ] 2s 2 2p 4 = 6 CI[1s 2 2s 2 2p 6 3s 2 3p 5 ] 3s 2 3p 5 = 7 Na[1s 2 2s 2 2p 6 3s 1 ] 3s 1 = 1 Mg[1s 2 2s 2 2p 6 3s 2 ] 3s 2 = 2 AL[1s 2 2s 2 2p 6 3s 2 3p 1 ] 3s 2 3p 1 = 3 ii) Ionization Energy (IE) or Ionization Potential (IP) is the energy required to remove one mole of electrons from one mole of gaseous atoms or ions e.g. Process Energy ev mol - 1 KJ mol -1 a)Na[1s 2 2s 2 2p 6 3s 1 ](g) Na + [1s 2 2s 2 2p 6 ] + e 5.10 495 2
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b) Al (g) Al + (g) + e 5.95 577 Al + (g) Al 2+ (g) + e 18.82 1816 Al 2+ (g) Al 3+ (g) + e 28.44 2745 This example illustrates that as the charge on the atom increases IE increases General Comments on IE In general metals have relatively low IE compared to non-metals In a period IE increases as the nuclear charge, z increases. In a Group IE decreases as z increases e.g. Li Li + + e, IE = 5200 KJ mol -1 Na Na + + e, IE = 495 KJ mol -1 K K + + e, IE = 418 KJ mol -1 Rb Rb + + e, IE = 403 KJ mol -1 iii) Electron Affinity (EA) is the energy change accompanying the addition of one mole of electrons to one mole of gaseous atoms or ions e.g. F (g) + e F(g) - , EA = -364 KJmol -1 Cl (g) + e Cl - (g), EA = -342 KJmol -1 I(g) + e I(g) -, EA = -295 KJmol -1 O(g) + e O - (g), EA = -141 KJmol -1 O - (g) + e O 2- (g), EA = +791 KJmol -1 S(g) + e S - (g), EA = -200 KJmol -1 S (g) + e S 2- (g), EA = + 649 KJmol -1 General Comments on EA In most cases, energy is liberated when the first electron is added because it is attracted to the atom’s nuclear charge, hence EA, is usually negative. EA 2 is always positive because energy must be absorbed to overcome electrostatic repulsive ion. In general non – metals have relatively high EA values than metals As a general trend EA increases as z increases in a Period.
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