Chem007Prelim2Rev - Chemistry 007, Fall 2007 Dr. Pontrello...

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Chemistry 007, Fall 2007 Dr. Pontrello Review Outline: Prelim 2, Chem 207 Chem 007 students from the most recent Weekly Group Session (11/4) expanded ( bulleted items) the review outline distributed in Chem 207. The expanded outline follows. Second Prelim: Thursday, November 8, 7:30-9:00pm Outline of Topics for Review Chemical Bonding (Chapter 10) Ionic, covalent and polar covalent bonds. • ionic – transfer of e-, combination of anions (negative charged) and cations (positive charged) held together by electrostatic forces (example: Na + Cl - ) result from removal of e- from the valence shell of the less electronegative atom • covalent – e- are shared (forms a bond), (example: Cl-Cl) • polar covalent – e- are not shared equally (example: H-Cl where H is δ + and Cl is δ - ) Lewis structures – the octet rule, assigning formal charge, exceptions to the octet rule (For the second period elements: molecules do not necessarily have to complete an octet; period 3 and higher molecules can expand their octet, e.g. SF 6 ). For period 3 or higher atoms, minimize formal charge at the expense of expanding the octet, Period 1 and 2 atoms cannot expand an octet. • all atoms except H react to obtain 8 e- (octet) in their valence shell (H has 2) • Formal Charge = (neutral valence e-) – [nonbonding e- + ½ bonding e-] • 2 nd period elements – NEVER expand octet • 3 rd period elements and higher can have expanded octets (minimize formal charges in Lewis structures – gives most accurate account of molecular structure) • exceptions to the octet rule 1) odd number e- (unpaired e-), example: H 3 C• 2) incomplete octets – most often the case with B, Al, Be, example: BF 3 3) expand valence shell, example: SF 6 Resonance Forms of a given molecule and their implication for bond length/strength • Situation where a single Lewis Structure does not accurately represent the e- distribution in the molecule • structures represent e- delocalization in a molecule (different structure ARE NOT different molecules) • show delocalized electrons through a pi-system (double bonds) • Best structure has full octets (if possible) and minimizes formal charges • Bond strength and length: for example, ozone (O 3 ), both bonds are the same length/strength. Resonance structures suggest 1.5 bond order (i.e. to accurately portray the molecule, both resonance structures are needed).
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Molecular shape and geometry, electron groups, VSEPR, relative repulsion of lone pairs and bond pairs. • refer to Table 10.1 (p. 399-400) in the text
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This note was uploaded on 02/28/2008 for the course CHEM 2070 taught by Professor Chirik,p during the Fall '05 term at Cornell University (Engineering School).

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Chem007Prelim2Rev - Chemistry 007, Fall 2007 Dr. Pontrello...

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