Chapter 9 Molecular Geometry and Bonding Theories

Trigonal Bipyramidal Electron Domain –Trigonal bipyramidal –Seesaw –T-shaped –Linear Table 9.3

Shapes of Larger Molecules Consider the geometry about a particular atom rather than the geometry of the molecule as a whole

Larger molecules tend to react at a particular site in the molecule Called a functional group Shapes of Larger Molecules acetic acid

Molecular Shape and Molecular Polarity A molecule possessing polar bonds does not imply that the molecule as a whole will be polar Fig 9.11 CO 2, a nonpolar molecule

To determine the overall dipole moment for the molecule, add the individual bond dipoles vectorially Molecular Shape and Molecular Polarity Fig 9.12

Molecular Shape and Molecular Polarity Fig 9.13 Molecules containing polar bonds Polar Nonpolar Polar

Valence bond theory – bonds are formed by sharing of e − from overlapping atomic orbitals (AOs) Overlap of:2 1s orbitals How does Lewis theory explain the bonds in H 2 and HCl? “Sharing of two electrons between the two atoms ” Covalent Bonding and Orbital Overlap 1s orbital and 3p orbital

Fig 9.15 Formation of the H 2 molecule 74 pm

Hybrid Orbitals VSEPR theory allows prediction of molecular shapes How can tetrahedral, trigonal bipyramidal, and other geometries arising from the atomic orbitals we recognize?

Hybridization – mixing of two or more atomic orbitals to form a new set of hybrid orbitals. 1.Mix at least 2 nonequivalent atomic orbitals (e.g. s and p). Hybrid orbitals have very different shape from original atomic orbitals. 2.Number of hybrid orbitals = number of pure atomic orbitals used in the hybridization process. 3.Covalent bonds are formed by: a)Overlap of hybrid orbitals with atomic orbitals b)Overlap of hybrid orbitals with other hybrid orbitals

sp Hybrid Orbitals F F Be VSEPR predicts: Linear, 180° Be Assume Be absorbs the small amount of energy needed to promote an electron from the 2s to the 2p orbital: it can form two bonds.

sp Hybrid Orbitals F F Be VSEPR predicts: Linear, 180° Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals: –These sp hybrid orbitals have two lobes like a p orbital. –One of the lobes is larger and more rounded as is the s orbital. Fig 9.16 formation of sp hybrid orbitals

These two degenerate orbitals would align themselves 180  from each other This is consistent with the observed geometry of beryllium compounds: linear sp Hybrid Orbitals Fig 9.17 Formation of two equivalent Be-F bonds in BeF 2

Using a similar model for boron leads to… sp 2 Hybrid Orbitals Fig 9.18

With carbon we get… sp 3 Hybrid Orbitals Fig 9.19

For geometries involving expanded octets on the central atom, we must use d orbitals in our hybrids : Hybridization Involving d Orbitals

This leads to five degenerate sp 3 d orbitals… …or six degenerate sp 3 d 2 orbitals. Hybridization Involving d Orbitals

# of Lone Pairs + # of Bonded Atoms HybridizationExamples sp sp 2 sp 3 sp 3 d sp 3 d 2 BeCl 2 BF 3 CH 4, NH 3, H 2 O PCl 5 SF 6 How do I predict the hybridization of the central atom? Count the number of lone pairs AND the number of atoms bonded to the central atom

Sigma () Bonds  Characterized by:  Head-to-head overlap  Single bonds are always  bonds Fig 9.14

Pi () Bonds  Pi bonds are characterized by:  Side-to-side overlap In a multiple bond: one of the bonds is a  bond and the rest are  bonds Fig 9.22

Multiple bonds in ethylene Fig 9.23 Molecular geometry of ethylene Fig 9.24 The σ bonds in ethylene sp 2

Multiple Bonds

The π bond in ethylene Fig 9.25