Shapes of molecules, hybrid orbitals and symmetry descriptionsSlide Number 2Vocabulary and ConceptsResonance Structures: A Way to Delocalize Electrons in Valence Bond DescriptionsHypervalenceGeometries may be predicted via VSEPR: Valence Shell Electron Pair RepulsionSlide Number 7Slide Number 8Slide Number 9Slide Number 10Slide Number 11Slide Number 12SymmetrySymmetry: mirror planesSymmetryChirality: Non-superimposable mirror images (aka optical isomers or enantiomers)Symmetry Operations and Symmetry ElementsSlide Number 18Slide Number 19Slide Number 20Slide Number 21Slide Number 22Slide Number 23Slide Number 24Slide Number 25Slide Number 26Slide Number 27Center of Inversion in Molecules: x,y,z -> -x, -y, -z(Proper) Rotation in ChemistryReflection in a Plane in ChemistrySlide Number 31Slide Number 32Slide Number 33Slide Number 34Slide Number 35Slide Number 36Shapes of molecules, hybrid orbitals and symmetry descriptions Lectures 10/11 2017 362 Spring term Some of these ppt slides from Dr. Oleg Ozerov’s lecture in 2014Lewis Structures  A bond between two atoms is formed by means of sharing of a pair of electrons  Each atom shares electrons with neighbors to achieve a total of eight valence electrons  Determine connectivity of the atoms in the molecule  Sum up the total number of valence electrons in the molecule  Distribute the electrons so that each atom acquires an octet (duet for H!) in either a) bonding pairs (denoted : or – ) shared between a pair of atoms, or b) lone pairs (denoted : ) that belong to a single atom (i.e., “unused” in making bonds and occupy more space than bonded pairs). Examples: HF, CF4, NH3, COCl2, CO, CO2, N2O, H2CN2, N3-, N5+Vocabulary and Concepts Parkin, G. J. Chem. Educ. 2006, 83, 791 Jensen, W. B. J. Chem. Educ. 2006, 83, 1751 Oxidation State or Number: Charge on atoms according to a set of rules That consider the electronegativity of atoms within the molecule or material. 1) In pure element, Oxidation Number = 0 2) F, the most electronegative element, in a molecule is -1 3) O is typically -2; sometimes (in peroxides), -1 4) Alkali metals, +1; Alkaline Earth metals, +2; Gp 3, generally +3; Transition metals variable + charged. 5) H is +1 when combined with more electroneg. element; -1 when combined with more electropositive element. Therefore, H in compound with any M is a hydride, H-1. 6) Summation of Ox. States must equal charge on ion; or zero if neutral molecule. Valence: Number of electrons an atom uses in bonding. Formal Charges: Charge on atom according to equation: F.C. = # valence electrons - # of electrons in non-bonded pairs – ½ # of electrons in bonds Coordination number: Number of atoms bonded to the atom in question.Resonance Structures: A Way to Delocalize Electrons in Valence Bond Descriptions  Resonance structures represent different Lewis structures for the same molecule  Resonance structures must have the same connectivity and the same total no. of electrons  Resonance structures are not isomers, but are various limiting descriptions of the same molecule.  If a molecule can be represented by more than one viable resonance structure, its electronic structure should be thought of as a “blend” of the resonance structures. Such “blending” generally lowers the energy of the system.  Resonance structure may be of equal or unequal importance (“weight”).  Guiding principles: Favor octets; minimize formal charges*; disfavor formal positive charge on more electronegative atoms S OCCH3OSCCH3*Pauling’s Electroneutrality PrincipleHypervalence  Hypervalence may be a misnomer by some definitions, but it would probably be the most commonly used misnomer in chemistry  Aka “octet expansion”  Can be represented either by an increased number of atoms bound to the “hypervalent” atom or by an increased number of (multiple) bonds to the “hypervalent” atom. Applies to 3rd row and higher non-metals bonds to highly electronegative elements. F PFFFFOPOOOGeometries may be predicted via VSEPR: Valence Shell Electron Pair Repulsion  Electron pairs of bonds and lone pairs repel each other  The geometry around any atom is a consequence of minimizing these repulsive interactions  Lone pairs are considered to be larger than bonding pairs  Multiple bonds are considered to be in the same space as, but larger than, single bonds  Several common geometries depending on the number of “occupants” around the atom in question  The “Steric number” of the molecule determines the hybrid orbitals used to account for sigma bonds and lone pairsDescriptions of geometries in mono- centric molecules: It is where The atoms are. . .Hybrid orbitals sp = s + pz sp2 = s + px + py sp3 = s + px + py + pz sp3d = (s + px + py) + (pz + dz2) sp3d2 = s + px + py + pz + dx2 - y2 + dz2VSEPR rationalizes bond angles and geometry of molecules For H2O and NH3, lone pairs are in sp3 hybrid orbitals; for H2S and PH3 No hybrid orbitals needed. For SF4 the lone pair is more stable in the sp2 subset of the sp3d hybrid orbitalsHow about the 5 pairs of electrons about Cl in ClF3? Analysis of lone pair/lone pair vs. lone pair/bonded pair vs. bonded pair/bonded pair repulsions Conclusion: larger pairs of electrons adopt equatorial plane positions, i.e., sp2 subsetSymmetry Intuitively, we know symmetry when we see it. But how do we put in quantitative terms that allows us to compare, assign, classify?Symmetry: mirror planesSymmetry C3 Rotation AxisChirality: Non-superimposable mirror images (aka optical isomers or enantiomers) Specifically, a chiral compound can contain no improper axis of rotation (Sn), which includes planes of symmetry and inversion center. Asymmetric molecules are always chiral.Symmetry Operations and Symmetry Elements Definitions:  A symmetry operation is an operation on a body such that, after the operation has been carried out, the result is indistinguishable from the original body (every point of the body is coincident with an equivalent point or the same point of the body in its original orientation).  A symmetry element is a geometrical entity such as a line, a plane, or a point, with respect to which one or more symmetry operations may be carried out Symmetry Operation Symmetry Element Notation Identity - E Reflection in a plane Plane