Ch_10_review - Chem 125 Dr Retsek Ch 10 Review of Lewis...

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Chem 125 Dr. Retsek Ch. 10 Review of Lewis Structures, Molecular Geometries, and Polarity How to draw a Lewis Structure: (a 2-dimensional picture of a molecule) ( Review section 10.1, pages 366-375. ) 1. Count all valence electrons contributed by all atoms of the molecule. ( Review how to determine number of valence electrons p. 302.) a. If a cation, subtract the charge of the ion from the total number of electrons. b. If an anion, add the charge of the ion to the total number of electrons. 2. Establish how atoms are connected a. Least electronegative atom in center ( Review electronegativity and the trend p. 344-346.) b. Hydrogen is never in center 3. Draw a single bond from each central atom to surrounding atom. Subtract 2 electrons for each bond. 4. Distribute remaining electrons (in pairs) so that each atom obtains an octet (8 electrons). Put the electrons on more electronegative atoms (the outer atoms) first, then the central atom. 5. If central atom does not have an octet when all electrons are distributed, make a multiple bond (double or triple bond) by moving a lone pair of electrons on a surrounding atom into a bonding pair. Examples: NF 3 COH 2 CO 3 2- Step 1: N: 5 electrons C: 4 electrons C: 4 electrons F: 7 electrons x 3 = 21 O: 6 electrons O: 6 electrons x 3 = 18 total: 26 electrons H: 1 electron x 2 = 2 charge: +2 more electrons total: 12 electrons total: 24 electrons Step 2: N least electronegative C least electronegative C least electronegative Step 3: 26 total e - – 6 e - in bonds 12 total e - – 6 e
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