Shows how valence electrons are arranged among atoms in a molecule. Reflects central idea that stability of a compound relates to noble gas electron configuration. Lewis Structures
Completing a Lewis Structure -CH3Cl • Make carbon the central atom • Add up available valence electrons: • C = 4, H = (3)(1), Cl = 7 Total = 14 • Join peripheral atoms • to the central atom • with electron pairs. H .. .. .. H .. • Complete octets on • atoms other than • hydrogen with remaining • electrons C .. Cl .. .. H
Multiple Covalent Bonds:Double bonds Ethene Two pairs of shared electrons
Multiple Covalent Bonds:Triple bonds Ethyne Three pairs of shared electrons
Resonance • Resonance is invoked when more than one valid Lewis structure can be written for a particular molecule. Benzene, C6H6 • The actual structure is an average of the resonance • structures. • The bond lengths in the ring are identical, and • between those of single and double bonds.
Resonance Bond Length and Bond Energy • Resonance bonds are shorter and stronger than single bonds. • Resonance bonds are longer and weaker than double • bonds.
Resonance in Ozone, O3 Neither structure is correct. Oxygen bond lengths are identical, and intermediate to single and double bonds
Resonance in Polyatomic Ions Resonance in a carbonate ion: Resonance in an acetate ion:
Localized Electron Model Lewis structures are an application of the “Localized Electron Model” L.E.M. says: Electron pairs can be thought of as “belonging” to pairs of atoms when bonding Resonance points out a weakness in the Localized Electron Model.
Models are attempts to explain how nature operates on the microscopic level based on experiences in the macroscopic world. Models Models can be physical as with this DNA model Models can be mathematical Models can be theoretical or philosophical
A model does not equal reality. Models are oversimplifications, and are therefore often wrong. Models become more complicated as they age. We must understand the underlying assumptions in a model so that we don’t misuse it. Fundamental Properties of Models
MOLECULAR GEOMETRY Molecule adopts the shape that minimizes the electron pair repulsions. VSEPR • Valence Shell Electron Pair Repulsion theory. • Most important factor in determining geometry is relative repulsion between electron pairs.
VSEPR – Valence Shell Electron Pair Repulsion A = central atom X =atoms bonded to A E = nonbonding electron pairs on A
VSEPR: Linear AX2 CO2
AX2 - Linear • AX - Linear • Draw CO2 and HF
BF3 AX3 Triganol planar AX2E SnCl2 Bent
VSEPR: Tetrahedral AX4 CCl4 tetrahedral AX3E PCl3 Triangular Pyramidal AX2E2 Cl2O Bent
VSEPR: Trigonal Bi-pyramidal AX5 PCl5 Triangular bipyramidal AX4E SF4 See-saw AX3E2 ClF3 T-shaped AX2E3 I3- Linear
VSEPR: Octahedral AX6 SF6 Octahedral AX5E BrF5 Square pyramidal AX4E2 ICl4- Square planar
Are the following molecules polar or nonpolar ? Which will dissolve in water? • OF2 SO3 PCl3 SF6 NH3
Formal charge (FC) in Lewis structures Comparison of the formal number of valence electrons about an atom in the Lewis structure of a molecule and comparison of FN with the number of valence electrons (VE) in the neutral atom. Computation of FC: FC = VE (neutral atom) - LE (atom in molecule) - 1/2BE (atom in molecule) where VE = the number of valence electrons in the neutral atom LE = the number of lone pair electrons on the atom in the molecule BE = the number of bonding electrons on the atom in the molecule
Example of formal charge computation: ozone, O3 VE (atom) = 6 6 6 1/2 BE (molecule)= -2 -3 -1 LE (molecule) = -4 -2 -6 FC = 0 +1 -1 Note: New charge for molecule = 0, sum of formal charges must = 0.
Isomers: Same composition, two different constitutional Lewis structures HCN = atomic compositional structure HCN possesses 10 VE = Lewis compositional structures Two possible Lewis constitutional structures: H-C-N or H-N-C Both need to have 10 VE in their Lewis structure Problem: Try to achieve an acceptable Lewis structure (duet and octet rule followed) for both.
HCN = atomic compositional structure HCN: 10 VE = Lewis compositional structures Two possible Lewis constitutional structures H-C-N or H-N-C Any acceptable Lewis structure for HCN needs to show 10 VE Try to achieve an acceptable Lewis structure (duet and octet rules obeyed) for all isomeric structures. Two acceptable Lewis structures. Which is better?
Use formal charges to decide on the stability of isomeric Lewis structures VE (atom) 1 4 5 1 5 4 1/2 BE (molecule) -1 -4 -3 -1 -4 -3 UE (molecule) 0 0 -2 0 0 -2 ________________________________________________________ FC on atom 0 0 0 0 +1 -1 Important: the net charge of composition HCN = 0, so the sum of the formal charges in any acceptable Lewis structure must be = 0 also.