lecture9 - Chem 1A Lecture 9: Exceptions Exception to Octet...

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Exceptions Chem 1A Lecture 9:
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Exception to Octet rule Octet rule applies mainly to second period elements. For other elements, many exceptions exist, due to: There are not enough valence electrons available in the structure to fill the octet on each atom. Octet rule is based on filling s and p shells (2+6 = 8 positions). Many atoms have d shells available and thus more positions to fill electrons.
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Incomplete octet Some atoms lack the number of valence electrons to fill the shell. They form stable compounds with less than eight electrons surrounding them. Typical elements: Be, B, Al Be: 1s 2 2s 2 Be Cl Cl 2 + (2 x 7) = 16 e - 8e - 8e - 4e - Be Cl Cl 8e - 8e - 8e - Formal charge: Cl : 7 - 6 = +1 Be : 2 - 4 = -2 + + 2- Highly unlikely!
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Incomplete Octet B: 1s 2 2s 2 2p 1 B F F F 3 + (3 x 7) = 24 e - Formal charge: F : 7 - 7 = 0 B : 3 - 3 = 0 B F F F Formal charge: F : 7 - 6 = +1 B : 3 - 4 = -1 + - Unlikely Lewis structure, but somewhat stabilized by resonance
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Incomplete Octet Al: [Ne]3s 2 3p 1 Al Cl Cl Cl 3 + (3 x 7) = 24 e - Formal charge: Cl : 7 - 7 = 0 Al : 3 - 3 = 0 Al Cl Cl Cl Al Cl Cl Cl Al Cl Cl Cl Al Cl Cl Cl Formal charge: Cl : 7 - 6 = +1 Al : 3 - 4 = -1 + + - - Coordinate covalent bond: bond formed from lone pair originating from one of the atoms.
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Odd-Electron Molecules Molecules with odd number of electrons cannot fulfill the
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lecture9 - Chem 1A Lecture 9: Exceptions Exception to Octet...

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