Structure and Bonding

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

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

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

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.

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

Writing Lewis Dot Structures 1.Write the correct molecular formula 2.Count the valence electrons for all the atoms Add 1 electron for each negative charge Subtract 1 electron for each positive charge 3.Create a skeletal structure 4.Add pairs of electrons for bonds Subtract bonding electrons from the pool 5.Add lone pairs 6.Make multiple bonds, if necessary

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

Writing Lewis Dot Structures Write the Lewis structure for dichloromethane CH 2 Cl 2 3.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

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

Writing Lewis Dot Structures Write the Lewis structure for dichloromethane CH 2 Cl 2 3.Create a skeletal structure CHClH

Writing Lewis Dot Structures Write the Lewis structure for dichloromethane CH 2 Cl 2 4.Add a pair of electrons for bonds between each bonded atom pair C H Cl H.. ::

Writing Lewis Dot Structures Write the Lewis structure for dichloromethane CH 2 Cl 2 5.Add pairs of electrons for lone pairs, as necessary C H Cl H.. :: ::

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 ed to you. At least one person in each group should bring a computer to lab