1 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Section 10 Molecular Geometry and Bonding Theory

2 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Bond Theory In this chapter we will discuss the geometries of molecules in terms of their electronic structure. –Valence Bond Theory (hybridization) allows for the construction of new orbitals on atoms so that the bonding in a molecule is made to be consistent with its known geometry. Bonding made possible by overlapping of orbitals (s, p, d, f  sp, sp 2, etc.) –Valence Shell Electron Pair Repulsion Theory (molecular geometry) allows for the best geometry (general shape) for a molecule to be predicted. Most stable arrangement attained by minimizing repulsion forces.

3 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Valence Bond Theory Valence bond theory is an approximate theory to explain the covalent bond from a quantum mechanical view. –According to V.B. theory, a bond forms between two atoms when the following conditions are met. 1.Two atomic orbitals “overlap” 2.The total number of electrons in both orbitals is no more than two. :

4 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Be 1s 2 2s 2 ____ ____ “ground state” 1s 2s ____  ____ ____ ____ ____ valence state 2s 2s 2px 2py 2pz “excited state” hybridization process – 2s and 2px ____ ____ are mixed so as to form two “hybrid” sp sp(mixed) orbitals for better overlap (sp). Valence Bond Theory BeH 2 H : Be : H s sp sp s

5 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Hybrid Orbitals CH 4 C: 2s 2 2p 2 ___ ___ ______ 2s 2px 2py 2pz The bonding in carbon might be explained as follows: –Four unpaired electrons are formed as an electron from the 2s orbital is promoted (excited) to the vacant 2p orbital.

C atom (ground state) 2s 2p Energy 2s 2p 1s C atom (promoted)

7 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Hybrid Orbitals One bond on carbon would form using the 2s orbital while the other three bonds would use the 2p orbitals. –This does not explain the fact that the four bonds in CH 4 appear to be identical. –Valence bond theory assumes that the four available atomic orbitals in carbon combine to make four equivalent “hybrid” orbitals. –2s, 2px, 2py, 2pz mix to form 4 equivalent sp 3 orbitals ____ ____ ____ ____ sp 3 sp 3 sp 3 sp 3

8 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Hybrid Orbitals Hybrid orbitals are orbitals used to describe bonding that are obtained by taking combinations of atomic orbitals of an isolated atom. –In carbon case, a set of hybrids are constructed from one “s” orbital and three “p” orbitals, so they are called sp 3 hybrid orbitals. –The four sp 3 hybrid orbitals take the shape of a tetrahedron.

9 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. s sp 3

You can represent the hybridization of carbon in CH 4 as follows. C atom (promoted or excited state) Energy 1s 2p 2s sp 3 1s sp 3 1s C atom (hybridized state) C atom (in CH 4 ) C-H bonds

11 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Hybrid Orbitals Note that there is a relationship between the type of hybrid orbitals and the geometric arrangement of those orbitals. –Thus, if you know the geometric arrangement, you know what hybrid orbitals to use in the bonding description. –Molecular geometry is the general shape of a molecule, as determined by the relative positions of the atomic nuclei.

12 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. The Valence-Shell Electron Pair Repulsion Model The valence-shell electron pair repulsion (VSEPR) model predicts the shapes of molecules and ions by assuming that the valence shell electron pairs are arranged as far from one another as possible (minimize repulsion).

13 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Predicting Molecular Geometry The following steps and figures will help discern electron pair arrangements. 1.Draw the Lewis structure 2.Determine steric number – number of electron density regions around the central atom. Count lone pair, single, double, and triple bond as one region. 3.Determine geometry from memorized chart.

14 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Arrangement of Electron Pairs About an Atom 3 pairs Trigonal planar 2 pairs Linear 4 pairs Tetrahedral 5 pairs Trigonal bipyramidal 6 pairs Octahedral

15 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. 1 Electronic Group No. 2 Electronic Geometry 3 Angles, XAX 4 Hybrid- ization Unshared Pairs 5 Molecular Geometry 6 Sketch 7 Polarity 2linear180 o sp0linearno 3trigonal planar120 o sp 2 0trigonal planarno <120 o sp 2 1bentyes 4tetrahedral109.5 o sp 3 0tetrahedralno <109.5 o sp 3 1pyramidalyes <109.5 o sp 3 2bentyes

16 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. 1 Electronic Group No. 2 Electronic Geometry 3 Angles, XAX 4 Hybrid- ization Unshared Pairs 5 Molecular Geometry 6 Sketch 7 Polarity 5 trigonal bipyramidal 90 o 120 o 180 o sp 3 d0trigonal bipyramidalno <90 o <120 o <180 o sp 3 d1see-sawyes <90 o <180 o sp 3 d2T- shapedyes 180 o sp 3 d3linearno 6octahedral90 o, 180 o sp 3 d 2 0octahedralno <90 o sp 3 d 2 1square pyramidalyes 90 o, 180 o sp 3 d 2 2square planarno

17 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Predicting Molecular Geometry 1 Steric number equals the number of electron density regions. Single bond, double bond, triple bond, and lone pair all equal one electron density region. 2 Electronic Geometry gives the electron family which includes lone pairs. 3 Angles are affected by the type of electron density regions. Repelling forces are greater in LPLP>LPBP>BPBP (LP – lone pair, BP – bonding pair) and multiple bonds are greater than single bonds. 4 Hybridization can be easily determined by the Steric number and remembering s, p 3, d 5 5 Molecular Geometry gives the shape of molecules including just atoms not lone pairs. 6 Example sketch of electronic geometry around of central atom 7 Polarity is given for case when all the ligands are the same.

18 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Ex. XeF 2 EG, MG, angles, hybrid, polarity, 3D NH 3 H N HH :

19 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Multiple Bonding According to valence bond theory, only one hybrid orbital is needed for each bond (whether a single or multiple) and for each lone pair. –For example, consider the molecule ethene.

20 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Each carbon atom is bonded to three other atoms and no lone pairs, which indicates the need for three hybrid orbitals. –This implies sp 2 hybridization. –The third 2p orbital is left unhybridized and lies perpendicular to the plane of the trigonal sp 2 hybrids. –The following slide represents the sp 2 hybridization of the carbon atoms.

21 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. C atom (ground state) 2s 2p Energy sp 2 2p 1s C atom (hybridized) (unhybridized)

22 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Multiple Bonding To describe the multiple bonding in ethene, we must first distinguish between two kinds of bonds. –A  (sigma) bond is a “head-to-head” overlap of orbitals with a cylindrical shape about the bond axis. This occurs when two “s” orbitals overlap or “p” orbitals overlap along their axis. –A  (pi) bond is a “side-to-side” overlap of parallel “p” orbitals, creating an electron distribution above and below the bond axis.

23 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline.

24 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Multiple Bonding –Two of the sp 2 hybrid orbitals of each carbon overlap with the 1s orbitals of the hydrogens. –The remaining sp 2 hybrid orbital on each carbon overlap to form a  bond.

25 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Multiple Bonding The remaining “unhybridized” 2p orbitals on each of the carbon atoms overlap side-to-side forming a  bond. –You therefore describe the carbon-carbon double bond as one  bond and one  bond. –Bottom line: single bond –  double bond – 1  and  triple bond – 1  and 2 

26 Material was developed by combining Janusa’s material with the lecture outline provided with Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, Majority of figures/tables are from the Ebbing lecture outline. Ex. Acetylene C 2 H 2 How many  and  bonds? Multiple Bonding H – C C – H 1 sp-sp head-on overlap (C-C) and 2 s-sp head-on overlaps (H-C)  3  2 unhybridized side-on overlaps (C-C)  2  H – C C – H HW 68