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Structure and Bonding

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  1. Structure and Bonding

  2. Introduction Structure Determines Function Physical and chemical properties of a compound determined by 3-D structure

  3. Normal order of business… • Composition and formula • Lewis-dot structure • connectivity • Valence-Shell Electron-Pair Repulsion • 3-D structure • Valence Bond Theory • description of bonding

  4. Composition and Formula • Does the molecule exist? • For our discussion, • ignore ionic compounds • ignore transition elements • that leaves mostly non-metals

  5. Lewis Formalism • Strictly, not a theory of bonding • Simply a useful tool to get atom connectivity in molecular compounds • Old view: the octet rule • Modern view: Covalent bonds are sharing of valence electrons. • The octet generalization sometimes works.

  6. Writing Lewis Dot Structures • Simple-but-looks-complicated algorithm • Requires a little memorization • Works really well for main-group elements

  7. Writing Lewis Dot Structures • Write the correct molecular formula • Count the valence electrons for all the atoms • Add 1 electron for each negative charge • Subtract 1 electron for each positive charge • Create a skeletal structure • Add pairs of electrons for bonds • Subtract bonding electrons from the pool • Add lone pairs • Make multiple bonds, if necessary

  8. Writing Lewis Dot Structures Write the Lewis structure for dichloromethane CH2Cl2 1. Count valence electrons Get ‘em from the periodic table C = 4 H = 1 (x 2) = 2 Cl = 7 (x 2) = 14 Total = 20

  9. Writing Lewis Dot Structures Write the Lewis structure for dichloromethane CH2Cl2 • Create a skeletal structure Some generalizations: H is almost always terminal with 1 bond Halogens are terminal except in polyatomic ions O and S generally have 2 bonds (1 is common in ions) N generally has 3 bonds (4 is common in ions) C generally has 4 bonds Usually, the least electronegative atom is surrounded by the more electronegative atoms

  10. Table of Electronegativies Usually, the least electronegative atom is surrounded by the more electronegative atoms

  11. Writing Lewis Dot Structures Write the Lewis structure for dichloromethane CH2Cl2 • Create a skeletal structure C Cl H H Cl

  12. Writing Lewis Dot Structures Write the Lewis structure for dichloromethane CH2Cl2 4. Add a pair of electrons for bonds between each bonded atom pair H .. : : Cl C Cl .. H

  13. Writing Lewis Dot Structures Write the Lewis structure for dichloromethane CH2Cl2 5. Add pairs of electrons for lone pairs, as necessary H .. .. .. : : : : .. Cl C .. Cl .. H

  14. This Week… • Not a formal report • Draw structures in lab manual. Transfer to a well-designed table with all required information. Add post-lab questions to back and turn all of that in at the next lab. • Identification of hybridization not required • Don’t forget the erratum emailed to you. • At least one person in each group should bring a computer to lab