resonance_and_formal - Electron Bookkeeping Lewis Dot and...

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Unformatted text preview: Electron Bookkeeping: Lewis Dot and General Structure, Formal Charge and Resonance Contributors Dr. Gergens - SD Mesa College Determining formal charges on individual atoms in Lewis structures and drawing resonance contributors are important elements in good "electron bookkeeping." So much of organic chemistry can be made more understandable by keeping track of electrons that it is worth taking some time at the beginning to become proficient at the seemingly simple task of counting electrons. This project is designed to review your current knowledge of electron counting used in drawing Lewis dot chemical structure and to develop your skill in calculating formal charge about individual atoms when drawing acceptable resonance structures. Before discussing formal charge and resonance, let's review some fundamentals for drawing Lewis dot structures. The covalent bond or shared electron pair model of chemical bonding was first suggested by G.N. Lewis of the University of California in 1916. Lewis proposed that the sharing of electrons by two nonmetal atoms permits each atom to have a stable closed-shell electron configuration. In forming compounds, atoms gain, lose, or share electrons to achieve a stable electron configuration characterized by eight valence electrons as in a noble gas. When the octet rule is satisfied for carbon, nitrogen, oxygen and fluorine they have an electron configuration analogous to the noble gas neon. Representing a 2-electron covalent bond by a dash ( — ), the Lewis structures for the most stable bonding modes for carbon, nitrogen, oxygen and hydrogen become: C H C H H O H H C H C H C O H H C H N H N H H H N N N H H O H H C H O C N H H H H H H H O H H (unstable) nitrogen gas diazene gas ammonia gas formaldehyde gas water liquid carbon dioxide gas acetylene gas ethylene gas methane gas •• •• •• •• •• •• •• •• •• •• •• •• : : : : : : : : : : : : : Multiple bonds are very common in organic chemistry, however, the most stable bonding mode for carbon is one where carbon has four covalent bonded dashes ( — ) around it. Ethylene (C 2 H 4 ) contains a carbon-carbon double bond in its most stable Lewis structure and each carbon has a completed octet. The most stable structure for acetylene (C 2 H 4 ) contains a carbon-carbon triple bond and the octet rule is satisfied. A systematic procedure for writing Lewis structures is outlined on the next page. 17 Summary of Steps for Drawing Lewis Dot Structures Dr. Gergens - SD Mesa College 1. Calculate the total number of valence electrons. 2. Assemble the bonding framework. 3. Connect the other atoms to the central by drawing a single line. Each line represents a single bond made up of two electrons being shared between two atoms....
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This note was uploaded on 09/09/2011 for the course CHEM 231l taught by Professor Gergens during the Spring '11 term at Mesa CC.

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resonance_and_formal - Electron Bookkeeping Lewis Dot and...

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