Formal_charge - 1 2 3 Formal Atom Charges Formal Atom...

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Unformatted text preview: 1 2 3 Formal Atom Charges Formal Atom Charges • Atoms in molecules often bear a charge (+ or -). • The predominant resonance structure of a molecule is the one with charges as close to 0 as possible. Carbon Dioxide, CO2 Carbon Dioxide, CO2 6 - (1/2)(4) - 4 = 0 •• •• Partial Charges in CO 2 • Formal charge = Group no. - 1/2 (no. bond electrons) - (no. of LP electrons) • • O C O • • 4 - (1/2)(8) - 0 = 0 Yellow = negative & red = positive Relative size = relative charge 4 5 6 Thiocyanate Ion, SCN5 - (1/2)(4) - 4 = -1 • • Thiocyanate Ion, SCN- Partial Charges in SCN- 6 - (1/2)(4) - 4 = 0 •• S •• •• C • • N S • • • • S N •• •• • • •• C N •• • • • • S •• C N • • C 4 - (1/2)(8) - 0 = 0 Which is the most important resonance form? All atoms negative — but most on the S Page 1 7 8 Boron Trifluoride, BF3 Boron Trifluoride, BF3 Trifluoride, • • •• Partial Charges in BF 3 F •• • • • • •• F • • +1 -1 • • F •• B • • • • F •• B • • F •• • • F •• • • What if we form a B—F double bond to satisfy the B atom octet? F is negative and B is positive Page 2 ...
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