Here these two structures contribute equally the

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Here, these two structures contribute equally . The double-headed arrow indicates the molecule is a hybrid of the contributing resonance structures. O O CH 3 _ O O CH 3 _ Some Rules for Drawing Resonance Structures Must be valid Lewis structures Never move atoms, only electrons Usually, only π or non-bonded electrons move Each must have same number of paired and unpaired electrons (e.g., for radical species) Some individual structures can be more important
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4 Some Rules for Assessing the Relative Importance of Different Resonance Structures Follow the octet rule when possible; however, non-octet structures may be valid if the resulting formal charges are consistent with the electronegativity differences. More covalent bonds are better Charge separation requires work Other things being equal, place the negative charge on the more electronegative atom Distorted bond angles or lengths are unstable Never place like charges on adjacent atoms (Formal Charge = # valence e - - # of bonds - # of non-bonding e - (lone pairs)) Formal Charges First: Normal Covalent Bonding Patterns C C C C Carbon Nitrogen N N N Oxygen O O Halogens X Four bonds No lone pairs Three bonds One lone pair Two bonds Two lone pairs One bond Three lone pairs
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5 Common Structures with Formal Charges C C Carbon Nitrogen N N N Oxygen O O Halogens X N N N O (Formal Charge = # valence e - - # of bonds - # of non-bonding e - (lone pairs)) Molecular Orbital Theory Use all Atomic Orbitals to build Molecular Orbitals
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