1Chem C1403 Lecture 6. Lewis structures and thegeometry of molecules with a central atom.(1) Covalent bonding: sharing of electron pairs by atoms(2) Rules for writing valid Lewis structures(3) Multiple bonding(3) Formal charges within Lewis structures(4) Isomers(5) Limitations of Lewis structures: resonance structuresand violations of the octet rule (odd electron, hypovalentand hypervalent molecules).2Beyond Lewis structures: the geometry of molecules(1) Shapes of simple molecules with “central” atoms.(2) Configuration: the position of atoms in space about a central atom. The VSEPR theory of configuration.(3) Steric number rules.(4) The five favored electronic geometries of configurations(5) Connecting molecular geometries to electronic geometries by VSEPRs(6) Polar covalent bonds: using molecular geometries to deduce the dipole moments of molecules3Writing acceptable Lewis structures(1) From composition to acceptable Lewis structures(2) Same composition, different constitution = isomers = differentmolecules(3) Same constitution, different Lewis structure = resonancestructures = one molecule different electronic representations(4) Comparing stabilities of isomers and resonance structuresthrough computation of formal charge for Lewis structuresImportant: Lewis structures indicate the way bondsconnect the atoms of a molecule, but they do notshow the three-dimensional molecular geometry.4A strategy for writing Lewis structures (others are possible).(1) Compute the number of valence electrons from the composition.Add one more electron for each negative charge in thecomposition. Subtract one electron for each positive charge in thecomposition.(2) All acceptable Lewis structures must have the correct compositionof atoms and charge.(3) An atomic constitution must be assumed or given.(4) Start to build up the electronic constitution (Lewis structure) bygiving the outer atoms a duet (H) or octet (C, N, O, F).(5) Share electrons until an acceptable Lewis structure is achieved.5Example: ozone, O3(1) Compute the number of valence electrons from the composition. Add onemore electron for each negative charge in the composition. Subtract oneelectron for each positive charge in the composition: VE = 3x6 = 18(2) All acceptable Lewis structures must have the correct composition of atomsand charge: Net charge = 0(3) An atomic constitution must be assumed or given:(4) Start to build up the electronic constitution (Lewis structure) by giving theouter atoms a duet (H) or octet (C, N, O, F), then adding any remainingelectron pairs to the internal atoms:(5) Share electrons until an acceptable Lewis structure is achieved:O O OO O OO O OVE = 16VE = 18O O OAcceptable Lewis structure for O3:6Formal charge (FC) in Lewis structuresComparison of the formal number of valence electrons about anatom in the Lewis structure of a molecule and comparison of FNwith 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)whereVE = the number of valence electrons in the neutral atomLE = the number of lone pair electrons on the atom in the moleculeBE = the number of bonding electrons on the atom in the molecule7Example of formal charge computation: ozone, O3O O OVE (atom) = 6 6 61/2 BE (molecule) = -2 -3 -1LE (molecule) = -4 -2 -6__________________________________FC = 0 +1 -1Note: New charge for molecule = 0, sum of formalcharges must = 0.8Multiple bonding in Lewis structuresTwo atoms can share one pair of electrons (single bond) 2 pairs ofelectrons (double bond) or 3 pairs of electrons (triplet bonds)C2H6 (ethane)C2H4 (ethylene)C2H2 (acetylene)Rule: More bonds,shorter bonds: triplebond shorter thandouble bond; doublebond shorter thansingle bond.9HCN = atomic compositional structureHCN possesses 10 VE = Lewis compositional structuresTwo possible Lewis constitutional structures:H-C-N or H-N-CBoth need to have 10 VE in their Lewis structureProblem: Try to achieve an acceptable Lewis structure(duet and octet rule followed) for both.Isomers: Same composition, two differentconstitutional Lewis structures10HCN = atomic compositional structureHCN: 10 VE = Lewis compositional structuresTwo possible Lewis constitutional structuresH-C-N or H-N-CAny acceptable Lewis structure for HCN needs to show 10 VETry to achieve an acceptable Lewis structure (duet and octet rulesobeyed) for all isomeric structures.H C N H N CTwo acceptable Lewis structures. Which is better?11Use formal charges to decide on the stability ofisomeric Lewis structuresVE (atom) 1 4 5 1 5 4 1/2 BE (molecule) -1 -4 -3 -1 -4 -3UE (molecule) 0 0 -2 0 0 -2________________________________________________________FC on atom 0 0 0 0 +1 -1H C NH N CH C NH N CImportant: the net charge of composition HCN = 0, so thesum of the formal charges in any acceptable Lewis structuremust be = 0 also.12Which is more stable?H C NH N C0 0 0 0 +1 -1 Rule:For two isomeric acceptable Lewis structures, the onewith the least separation of formal charges is morestable.Therefore, is the morestable isomer of the pair.H C N13The same atomic composition can correspond to manyLewis acceptable structuresExample: C6H6This is the atomic composition of the famous organicmolecule, benzeneCCCCCCHHHHHHHow many other isomers (acceptable Lewisstructures) of C6H6 are possible?14Isomers of the composition C6H615Resonance structures: two or more Lewis electronicstructures for the same atomic compositionExample: ozone: atomic composition = O3Atomic constitution: O O OElectronic constitution:Neither Lewis structure is an accurate representation of theactual molecule: an equal mixture of the two structures is required:Both O-O bonds of O3 are equal in length.O O OO O O163.7 The Geometry (Shape) of Molecules: Valence ShellElectron Pair Repulsion (VSEPR) Theory(1) Computing the number of bonds and pairs of non-bondingelectrons on a central atom of a molecule(2) Predicting the shape of the disposition of atoms in spaceabout a central atom of a molecule from (1)(3) Dealing with dipole moments, polar bonds, non-polar bonds(4) Dealing with polar and non-polar molecules with polarbonds17The Valence shell electron pair repulsion (VSEPR)theory of the configuration (shape) of simplemoleculesLewis structures show how atoms are connected,not the angles or lengths of bonds in 3D.VSEPR allows the translation of a Lewis structureinto a configuration about a central atom byfollowing the rules of VSEPR