Ch13_VSEPR_061711 - Ch 13: Covalent Bonding Section 13:...

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Ch 13: Covalent Bonding Section 13: Valence-Shell Electron-Pair Repulsion 1. Recall the rules for drawing Lewis dot structures 2. Remember the special situations: - Resonance structures - Formal charges - Sub-octets and expanded octets 3. Use the best LDS to determine the electron pair geometry and molecular shape of the molecule/ion
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LDS Mechanics Three steps for “basic” Lewis structures: 1. Sum the valence electrons for all atoms to determine total number of electrons (add e- for any net negative charge or remove e- for any net positive charge) 2. Use pairs of electrons to form a bond between each pair of atoms (bonding pairs). 3. Arrange remaining electrons around atoms (lone pairs or multiple bonds) to satisfy the “octet rule” for each atom (“duet” rule for hydrogen). Remember
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LDS Rules of Thumb 1. In a polyatomic molecule, the atom that can make the most number of bonds typically goes in the center. This atom is also typically the least electronegative atom in the molecule. 2. H can only form one bond, so it goes on the outside of the molecule. ..H is a terminal atom. 3. If O and H both appear in a chemical formula, there’s a good chance that they are bonded to each other. 4. When several C atoms appear in the same formula, they are probably bonded to each other in a chain. In other situations the C atoms can form a closed loop, or branching structures…we’ll come back to that in Ch 21! =) Remember
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Beyond the Octet Rule • There are numerous exceptions to the octet rule. • We’ll deal with two classes of violation here: – Sub-octet systems (less than 8 electrons) – Valence shell expansion (more than 8 electrons) Remember
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Sub-Octet Systems • Some atoms (for example: Be, B, and Al) can form stable molecules that do not fulfill the octet rule. • Experiments demonstrate that the B-F bond strength is consistent with single bonds only. F B F F F B F F Remember
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Valence Shell Expansion For third-row elements (“Period 3”), the energetic proximity of the d orbitals allows for the participation of these orbitals in bonding.
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Ch13_VSEPR_061711 - Ch 13: Covalent Bonding Section 13:...

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