Chapter 9 Molecular Geometry and Bonding Theories

Molecular Shapes Shape of a molecule plays an important role in its reactivity The number of bonding and nonbonding electron pairs is used to predict the shape of the molecule Lewis structure of carbon tetrachloride, CCl 4

Fig 9.1 Tetrahedral geometry of CCl 4

Fig 9.2 Some common molecular shapes

Fig 9.3 Shapes of AB n molecules

Fig 9.3 Derivatives from AB n geometry CH 4 H2OH2ONH 3

What Determines the Shape of a Molecule? Electron pairs, whether they be bonding or nonbonding, repel each other By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the molecule

Electron Domains Electron pairs are referred to as electron domains. In a double or triple bond, all electrons shared between those two atoms are on the same side of the central atom; therefore, they count as one electron domain. Central atom, A, in this molecule, has four electron domains

Valence shell electron pair repulsion (VSEPR) model: The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them. Fig 9.5 Balloon analogy for electron domains

Table 9.1 Electron-domain geometries as a function of the number of electron domains

Fig 9.6 The molecular geometry of NH 3 Electron-domain geometry is often not the shape of the molecule, however. Molecular geometry is that defined by the positions of only the atoms in the molecules, not the nonbonding pairs.

The molecular geometry of CO 2 The molecular geometry of O 3 Linear; 180° Bent; 120°

Cl Be 2 atoms bonded to central atom 0 lone pairs on central atom BeCl 2

BF 3

CH 4

equatorial axial PCl 5

SF 6

Linear Electron Domain In the linear domain, there is only one molecular geometry: linear NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain Table 9.2

Trigonal Planar Electron Domain There are two molecular geometries: –Trigonal planar, if all the electron domains are bonding –Bent, if one of the domains is a nonbonding pair Table 9.2

Tetrahedral Electron Domain There are three molecular geometries: –Tetrahedral, if all are bonding pairs, –Trigonal pyramidal if one is a nonbonding pair, –Bent if there are two nonbonding pairs. Table 9.2

Nonbonding Pairs and Bond Angles Nonbonding pairs are physically larger than bonding pairs. Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule.

Multiple Bonds and Bond Angles Double and triple bonds place greater electron density on one side of the central atom than do single bonds: Therefore, they also affect bond angles.

Trigonal Bipyramidal Electron Domain There are two distinct positions in this geometry: –Axial –Equatorial

Trigonal Bipyramidal Electron Domain Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry. SF 4

Trigonal Bipyramidal Electron Domain There are four distinct molecular geometries in this domain: –Trigonal bipyramidal –Seesaw –T-shaped –Linear Table 9.3

Octahedral Electron Domain All positions are equivalent in the octahedral domain. There are three molecular geometries: –Octahedral –Square pyramidal –Square planar Table 9.3

Predicting Molecular Geometry 1.Draw Lewis structure for molecule. 2.Count number of lone pairs on the central atom and number of atoms bonded to the central atom. 3.Use VSEPR to predict the geometry of the molecule. What are the molecular geometries of SO 2 and SF 4 ? SO O AB 2 E bent S F F F F AB 4 E distorted tetrahedron