F fc b 3 0 ½6 0 f b f fc f 7 6 ½2 0 the formal

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F FC B = 3 – 0 – (½)(6) = 0 F B F FC F = 7 – 6 – (½)(2) = 0 The formal charges are more favorable for this structure. Case 3. Valence shell expansion Elements with n = or > 3 have empty ____ - orbitals, which means more than eight electrons can fit around the central atom. Expanded valence shells are more common when the central atom is ______ and is bonded to small, highly electronegative atoms such as O, F, and Cl. 3
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Consider PCl 5 Cl Cl P Cl 2) 5 + 5(7) = ______ valence electrons Cl Cl 3) 8 + 5(8) = ______ electrons needed for octet 4) 48 – 40 = ______ bonding e - s To make five P-Cl bonds, need ______ shared electrons. So 40 – 10 = 30 lone-pair electrons. Consider CrO 4 2- 2) 6 + 4(6) + _____ = ______ valence electrons 3) 8 + 4(8) = ______ electrons needed for octet 4) 40 – 32 = ______ bonding e - s 7) 32 – 8 = 24 lone-pair electrons. 8) calculate formal charges: FC Cr = 6 – 0 – (½)(8) = +2 FC O = 6 – 6 – (½)(2) = -1 Total charge = 2 + 4(-1) = -2 But experimentally, Cr-O bond length and strength are between that of a single and double bond! plus ________ other resonance structures. FC Cr = 6 – 0 – (½)12 = 0 FC ODB = 6 – 4 – (½)4 = 0 FC O = 6 – 6 – (½)2 = -1 Cr O O O O -2 Cr O O O O Cr O O O O -2 -2 4
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Valence shell expansion around Cr results in ________ formal charge separation. More stable Lewis structure. II. IONIC BONDS Ionic bonds involve the complete _____________ of (one or more) electrons from one atom to another with a bond resulting from the electrostatic attraction between the cation and anion. Consider the formation of NaCl from the neutral atoms, Na and Cl. Na (g) Na + (g) + e E = ______ = 494 kJ/mol Cl + e Cl (g) E = – EA = _____ kJ/mol Na (g) + Cl (g) Na + (g) + Cl (g) E = + ________ kJ/mol E = IE Na + (– EA Cl ) = 145 kJ/mol Problem: Na (g) + Cl (g) Na + (g) + Cl (g) has a positive E. It ___________ energy. Solution: Coulomb attraction. Na + (g) + Cl (g) NaCl (g) E = – 589 kJ/mol Net change in energy for the overall process: Na (g) + Cl (g) NaCl (g) E = __________ kJ/mol The mutual attraction between the oppositely-charged ions releases energy. The net energy change for the formation of NaCl is a decrease in energy. We can calculate the Coulomb attraction based on the distance between the two ions (assume here that the ions are point charges): z 1 z 2 e 2 for 2 unlike charges, U(r) = z = charge numbers of the ions and 4 π ε 0 r
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  • Fall '08
  • Vogel
  • Atom, Chemical bond

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