Lecture-14.doc

Lecture-14.doc - Chem111 Fall, 2005 Lecture 14: Bonding and...

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Chem111 Fall, 2005 51 Lewis Dot Structures accounting of all of the valence electrons – where do t h e y g o ? 1916, Gilbert Lewis bond involves a pair of electrons ± "shared" by the atoms ± why 2? why not 1 or 3 or 7? Lewis derives a simple method for predicting bonding in molecules Octet Rule : atoms want to have/share 8 electrons in their valence (outer) shells ± accounts for a wide range of stable molecules ± lots of exceptions ± manifestation of the stability of the inert/noble gas e- configuration ± F ± F- and Ca ± Ca 2+ because of this! Examples. Let's get back to CO 2 and H 2 O … why does CO 2 have double bonds b u t H 2 O only has single bonds? H 2 O each H contributes 1 valence e- = 2 e- the O contributes 6 valence e- = 6 e- T o t a l v a l e n c e e - = 8 e- first, draw the molecule in its bonding arrangement H O H then put in the bonding electrons – 2 per bond – leaves 8-4=4 e- to distribute H O H then put in the remaining 4 electrons … H O H Lecture 14: Bonding and Molecules Lewis Dot Structures VSEPR (I&II) ** see the Lewis Dot Structure Handout! ** two O-H single bonds
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Chem111 Fall, 2005 52 C O 2 each O contributes 6 valence e- = 12 e- the C contributes 4 valence e- = 4 e- T o t a l v a l e n c e e - = 16 e- draw the molecule in its bonding arrangement and put in e- pairs for bonds O C O 16 – 4 electrons ± 12 valence e- left to distribute distribute these 12 e- taking care to make sure each atom has 8 e- around it O C O Molecular Geometries ok … so now we know how to assign electrons to bonds and lone pairs – what do the molecules l o k i e ? Valence Shell Electron Pair Repulsion (VSEPR) and Molecular Orbitals (MO) We'll do VSEPR first (today) and MO over the next two lectures … VSEPR is based upon the idea that electrons in the valence shell repel one another.
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This note was uploaded on 02/08/2010 for the course CHEM 111 taught by Professor Kenney during the Spring '08 term at Case Western.

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Lecture-14.doc - Chem111 Fall, 2005 Lecture 14: Bonding and...

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