Chapter 9 Molecular Structure Under what rules do atoms form molecules? New materials, such as the lightweight gear used by these climbers, allow us to explore our world further than thought possible before. The development of new materials like these relies on the principles of molecular structure introduced in this chapter.
Assignment for Chapter 9 9.7, 9.12, 9.15, 9.21, 9.25, 9.28, 9.33 9.44, 9.54, 9.56
Some of the basic geometrical shapes that are used to describe the shapes of simple molecules
VSEPR Model(Valence-Shell Electron-Pair Repulsion model) Bonding electrons and lone pairs take up positions as far from one another as possible, for then they repel each other the least.
VSEPR Model (1): locate the high concentrations Electron pairs or bonds are as far away from one another as possible and so experience the minimal repulsion from other electrons The positions taken up by regions of high electron concentration (green) around a central atom.
Four ‘highs’ Two ‘highs’ Three ‘highs’ Five ‘highs’ Six ‘highs’ Seven ‘highs’ VSEPR Model (2): determine the shapes A summary of the positions taken up by regions of high electron concentration (other atoms and lone pairs) around a central atom. The locations of these regions are given by straight lines sticking out of the central atom. Use this chart to identify the arrangement of a given number of atoms and lone pairs and then use Fig. 9.2 to identify the shape of the molecule from the location of its atoms.
VSEPR Model: Example Electron pairs or bonds are as far away from one another as possible and so experience the minimum repulsion from other electrons Shape: linear (two ‘highs’) Shape: trigonal planar (three ‘highs’)
VSEPR Model: Example Electron pairs or bonds are as far away from one another as possible and so experience the minimum repulsion from other electrons Shape: trigonal bipyramidal (five ‘highs’)
VSEPR Model • In order to reduce repulsions, bonding pairs and lone pairs take up positions around an atom that maximize their separations. The shape of the molecule is determined by the location of the atoms attached to the central atom.
Classroom Exercise Predict the structure of SiCl4 and AsF5
F F AsF5 As F F F SiCl4 Classroom Exercise Predict the structure of SiCl4 and AsF5 Si: 3s23p2 As: 3d104s24p3
Molecules with Multiple Bonds The VSEPR model does not distinguish single and multiple bonds. A multiple bond is treated as just another region of high electron concentration. (Three ‘highs’) (Two ‘highs’)
The VSEPR model does not distinguish single and multiple bonds. A multiple bond is treated as just another region of high electron concentration. Two central atoms
Example Two central atoms, no lone pairs. The most possible VSEPR arrangement:
CH2O Formaldehyde CH2O
Classroom Exercise Predict the structure of HCN
How about if the central atoms have lone pairs? • A: central atom • X: atom bonded to the central atom • E: lone pair on the central atom Lone pairs on attached atoms have little effect on molecular shape, but the lone pairs on central atoms may have significant effect. The single nonbonding electron on radicals is treated as a ‘lone pair’.
Repulsion Order: Lone pair/lone pair > lone pair/bonding pair > bonding pair/bonding pair
Example H2O AX2E2 Shape: Angular (not tetrahedron!)
NH3 AX3E Triangular pyramid (NOT tetrahedron!)
NO2- Classroom Exercise Predict the shape of NO2- AEX2 Smaller angle Angular (NOT planar triangle!)
AX4E Axial lone pair Equatorial lone pair
AX4E2 Square planar (NOT octahedron!)
How to Use VSEPR Model 1. Write Lewis structure and determine the number of electron pairs 2. Maximize the separations. 3. Decide the positions of lone pairs (on the central atom). 4a. Name the shape (without considering the lone pair). 4b. Consider distortion using repulsion order. Lone pair/lone pair > lone pair/bonding pair > bonding pair/bonding pair
Example: SF4 AX4E Equatorial lone pair Bent seesaw T-shaped (NOT triangular bipyramid!)
ClF3 T-shaped (NOT triangular bipyramid!) AX3E
Classroom Exercise: XeF4 AX4E2 Square planar (NOT octahedron!)
Quiz • Write the VSEPR formula of water and sulfur tetrafluoride, draw and name their structures.
Answer • Write the VSEPR formula of water and sulfur pentafluoride. Draw and name their structures. AX2E2 AX4E T-shaped (not triangular bipyramid!) Angular (not tetrahedron!)
詩云 VSEPR 頌 Ode to VSEPR 中心邊緣孤對通， 三位一體電斥鬆。 主要形狀心中記， 孤對不在名字中。 Central, attached plus lone pairs found The trinity held by electrical repulsive bounds. Memorize the major molecular shapes and ignore the lone pair in naming the compounds.
Charge Distribution in Molecules Where does have high or low electron density (concentration)? Electron distribution is responsible for molecular properties and functions.
Polar Bonds r Partial positive charge Partial negative charge
1 Debye r=100 pm (=1 °A) +e -e μ=1 D
Polar Bonds Forming Nonpolar Molecules Total dipole moment = 0 Partial negative charge Partial negative charge Partial positive charge