Chapter 9 Notes

Chapter 9 Notes - Chapter 9: Chemical Bonding Ionic...

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1 Chapter 9: Chemical Bonding • Ionic compounds form when metals and nonmetals react • The attraction between positive and negative ions is called an ionic bond • The ionic compounds form because the potential energy of the system decreases • Consider the example of sodium chloride • The energy change when NaCl forms can be calculated using the ionization energy (IE) of sodium, the electron affinity (EA) of chlorine, and the lattice energy of NaCl The energy needed to produce separated gaseous ions from one mole of the solid is the lattice energy . • Starting from 1 mole of gas phase atoms: Na(g) Æ Na + (g) + e - +495.4 kJ (IE of sodium) Cl(g) + e - Æ Cl - (g) -348.8 kJ (EA of chlorine) Na + (g)+Cl - (g) Æ NaCl(s) -787.0 kJ (-lattice energy) Net: -640.4 kJ • It turns out that for any ionic compound, the chief stabilizing influence is the lattice energy • The size of the lattice energy depends on ion size and charge
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2 – The lattice energy increases with charge because the ions attract each other more strongly • Example: KCl (709 kJ) vs CaO (3401 kJ) – Smaller ions have larger lattice energies because they get closer together • Example: NaCl (778 kJ) vs KCl (709 kJ) and LiF (1033 kJ) vs LiCl (845 kJ) • The lattice energy can be calculated using a Born-Haber cycle • Noble gas configurations are very stable and can be useful in predicting ion charges – Consider the case of sodium: Na(g) Æ Na + (g)+e - IE= 466 kJ/mol Na + (g) Æ Na 2+ (g)+e - IE=4563 kJ/mol Na 1 s 2 2 s 2 2 p 6 3 s 1 Na + 1 s 2 2 s 2 2 p 6 (noble gas core) Na 2+ 1 s 2 2 s 2 2 p 5 – Formation of Na + is relatively inexpensive –N a 2+ doesn’t (ordinarily) form because breaking into the noble gas core costs a to much energy • All noble gases (except He) have 8 valence electrons • This is called an octet of electrons • Most of the representative elements tend to gain or lose electrons until they have achieved the configuration of the nearest noble gas • For example: Na and K lose electrons to achieve an octet of electrons while Cl and O gain electron to achieve an octet of
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3 • The octet rule works best for ionic compounds of Group IA and IIA metals from Period 3 down and for the anions of the nonmetals • It fails for Li and Be because they achieve the He (1 s 2 ) electron configuration • It also doesn’t work for hydrogen which can form H - (electron configuration: 1 s 2 ) when it reacts with very reactive metals The octet rule doesn’t work well for transition metals and post transition metals For these cations: 1) The first electrons lost by an atom or ion are always those from the outer shell (with the largest value of n ) 2) Within a given shell: the f (subshell) is emptied before the d , which is emptied before the p , which is emptied before the s
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This note was uploaded on 01/12/2012 for the course CHEM 103 taught by Professor Jeffashley during the Fall '10 term at Philadelphia.

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Chapter 9 Notes - Chapter 9: Chemical Bonding Ionic...

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