Directed Bonding Models: Lewis Dot and VSEPR

1. Directed Bonding Models: Lewis Dot and VSEPR • Lewis Dot Structures • Use dots to represent valence electrons. • Pairs of dots (lines) between atoms are bonds (multiple pairs of dots represent multiple bonds). • Valence electrons not involved in bonds are usually in pairs. • Each atom in a molecule bonds in such a way that it achieves a noble gas configuration (Octet ‘Rule”).

2. Writing Lewis Dot Structures • Determine number of valence electrons in molecule • cations (+) subtract electrons • anions (-) add electrons • Write skeleton structure for molecule • least electronegative element at the center • use formula a hint • H’s always, and Halides most often, terminal (NEVER form multiple bonds) • oxygens do not like to bond together (with a few exceptions) • oxygens most often have at least two sets of nonbonding pairs • Subtract 2 electrons for each bond in skeleton structure • Subtract 2 electrons for each lone pair added to structure • Distribute remaining valence electrons so that all atoms have complete octet • may need to introduce multiple bonds

3. SO3 Writing Lewis Dot Structures: Example O S O O

4. Exceptions to the Octet Rule • Elements of group IIIA form “electron deficient” compounds; have incomplete octet. BF3 • Elements with Z > 14 may have “expended octets” (10 or 12 electrons).

5. Formal Charges • :O: • :O N OH • simple means of bookkeeping • the + and - signs do not necessarily indicate actual charges • negative charge on more electronegative atom • sum of Formal Charges = charge on molecule or ion • Electroneutrality principle: formal Charges are to be avoided whenever possible • e.g. draw 2 structures for CHN, which actually exists? • H C N: H N C: F.C. = valence electrons - nonbonding electrons - bonds .. .. .. .. N = 5 - 0 - 4 = +1 O = 6 - 6- 1 = -1 + +

6. Resonance • in many cases there are multiple equivalent places to put multiple bonds: • O O • e.g. HNO3 O N OH O N OH • note: all N-O bond lengths are identical; but neither single nor double. • if 2 or more Lewis structures exist using the same backbone, then resonance is said to exist • the individual structures = contributing structures • the molecule itself is a resonance hybrid of the contributing structures • represent hybrid by drawing contributing structures and separating them by a • the resonance hybrid is a blend (average) of the contributing structures O • O N OH - - + + - bond lengths equivalent to a 11/2bond +

7. Molecular Shape • Valence Shell Electron Pair Repulsion Model (VSEPR) • Most Important Rule #1: • The bond angles around a central atom are those which maximize the distance between electron pairs in the valence shell of the atom • Procedure: • Draw Lewis structure • Count the number of electron pairs (both bonding and non-bonding) • multiple bonds as single “pair” • Determine Ideal Molecular Geometry • Make Corrections to Ideal Geometry • Nomenclature: • electronic geometry: how all electron pairs (bonding and non-bonding) are arranged about the central atom • molecular geometry: how all bonding pairs are arranged about that central atom

8. Electron Pair Geometry Ideal Molecular Geometry Electron Pairs Angle(s) Name Two (sp) 180o linear Three (sp2 ) 120o trigonal planar Four (sp3) 109.5o tetrahedral Five (sp3d) 120o & 90o trigonal bipyramidal Six (sp3d2) 180o octahedral

9. Ideal Molecular Geometry

10. Corrections to Ideal Molecular Geometry • Nonbonding Domains • nonbonding domains occupy more space than bonding domains • don’t “see” nonbonding domains, but do see their effects • repulsions: NBD-NBD > NBD-BD > BD-BD correction: H-N-H bond angle less than ideal ideal geometry: trigonal pyramidal electronic geometry: tetrahedral

11. Corrections to Ideal Molecular Geometry • Multiple Bonds • “count” as 1 bonding pair • larger than single bonds; take up more room • :O::O: • C C • Cl Cl Cl Cl • Electronegative Substituents bonding pairs to electronegative substituents occupy less space that those to more electropositive substituents. ideal geometry: trigonal planar correction: Cl-C-Cl bond angle less than ideal ●● P X X X X Θ F 97.8 Cl 100.3 Br 101.5 I 102 electronegativity Θ