Sept_21 - Formal Charges Formal charge = of valence e of...

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Formal Charges Formal charge = # of valence e - – ½(# of bonding e - ) – # of lone pair e - Example: HSCN (a) H = 1 - ½(2) - 0 = 0 S = 6 - ½(4) - 4 = 0 C = 4 - ½(8) - 0 = 0 N = 5 - ½(6) - 2 = 0 (b) H = 1 - ½(2) - 0 = 0 S = 6 - ½(6) - 2 = +1 C = 4 - ½(8) - 0 = 0 N = 5 - ½(6) - 2 = -1 H – S – C – N : ¨ ¨ ¨ ¨ (a) H-S-C≡N : vs (b) H-S=C=N ¨ ¨ ¨ ¨ ¨
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Formal Charges The sum of the formal charges must equal the total charge on the molecule or ion. Formal charges should be as small as possible. Negative formal charges usually are on the most electronegative atoms; positive formal charges on the least electronegative atoms. Structures having formal charges of the same sign on adjacent atoms are unlikely.
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Resonance Consider SO 2 : S O O S O O S O O S O O S O O or Which structure is correct?
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S O O S O O Which structure is correct? The true structure is somewhere in between the two. We call these
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Sept_21 - Formal Charges Formal charge = of valence e of...

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