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Ch3-final-CS - b Consider phase of wavefunctions 4 Valence...

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Chapter 3: VSEPR, dipole, VB, and MO theory 1) Valence shell electron repulsion theory Key: Electron pairs arrange themselves as far apart as possible, and this determines the shape. Candace’s method: a) Figure out how many e- pairs are on the atom (Lewis structure) b) Arrange as far apart as possible c) Add in lone pairs as appropriate (lone pairs are even more repulsive than bonding pairs) d) Pick the right name based on what’s visible Ex: Shape of I 3 - ? Ex: Bond angle of O 3 ? [Practice: all Exam 2 questions on shape and dipole; 3.3-14,17-18] 2) Dipole/Polarity A dipole is a charge separation. In covalent bonds, occurs when one atom is more EN than the other (pulls e- pair towards itself) Candace’s method: a) Figure out the shape (VSEPR) b) Consider each bond separately c) Sum vectors for overall effect Ex: NH 3 Ex: [Practice: review Exam 2, 3.19-24] 3) Some background on bonds a) A bond forms when orbitals from 2 different atoms overlap -> form a shared molecular orbital
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Unformatted text preview: b) Consider phase of wavefunctions 4) Valence Bond theory Normal orbitals don’t match with VSEPR. We need new orbitals. How to form? Take combinations of old orbitals! a) One orbital needed for every pair of electrons - σ bond - lone pair b) # orbital in = # orbitals out Use lowest energy orbitals c) σ bonds = 1 lobe overlap; π = 2 lobes overlap Orbitals available for carbon: Example: Compare… Ethane, CH3-CH3 Ethene, CH2=CH2 Ethyne,HCΞCH [Practice: check Exam2; 3.35-40] 5) Molecular orbital theory Also concerned with orbital overlap to form bonds, but we use the normal orbitals. Two questions: Bond order? Paramagnetic? Candace’s method: a) Choose (and USE) the right diagram b) Count total # of electrons c) Fill molecular orbitals according to Aufbau (Ch 1) [Practice: Exam 2 paramagnetism question; 3.49-53]...
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