CH 10 Molecular Geometry Renee Y Becker Valencia

CH 10: Molecular Geometry Renee Y. Becker Valencia Community College CHM 1045 1

Molecular Shapes: VSEPR • The approximate shape of molecules is given by Valence-Shell Electron-Pair Repulsion (VSEPR). • Step 01: Count the total electron groups. • Step 02: Arrange electron groups to maximize separation. • Groups are collections of bond pairs between two atoms or a lone pair. • Groups do not compete equally for space: Lone Pair > Triple Bond > Double Bond > Single Bond 2

Molecular Shapes: VSEPR • Two Electron Groups: Electron groups point in opposite directions. 3

Molecular Shapes: VSEPR • Three Electron Groups: Electron groups lie in the same plane and point to the corners of an equilateral triangle. 4

Molecular Shapes: VSEPR • Four Electron Groups: • Electron groups point to the corners of a regular tetrahedron. 5

Molecular Shapes: VSEPR Five Electron Groups: Electron groups point to the corners of a trigonal bipyramid. 6

Molecular Shapes: VSEPR • Six Electron Groups: Electron groups point to the corners of a regular octahedron. 7

Molecular Shapes: VSEPR 8

Example 1: VSEPR • Draw the Lewis electron-dot structure and predict the shapes of the following molecules or ions: O 3 H 3 O + Xe. F 2 PF 6– Xe. OF 4 Al. H 4– BF 4– Si. Cl 4 ICl 4– Al. Cl 3 9

Valence Bond Theory 1. Covalent bonds are formed by overlapping of atomic orbitals, each of which contains one electron of opposite spin. 2. Each of the bonded atoms maintains its own atomic orbitals, but the electron pair in the overlapping orbitals is shared by both atoms. 3. The greater the amount of orbital overlap, the stronger the bond. 10

Valence Bond Theory • Linus Pauling: Wave functions from s orbitals & p orbitals could be combined to form hybrid atomic orbitals. 11

• sp hybrid: 12

• sp 2 hybrid: 13

• sp 2 hybrid (π bond): 14

• sp 3 hybrid: 15

sp 3 d hybrid: 16

• sp 3 d 2 hybrid: 17

Hybridization Easy Way 18

Molecular Orbital Theory • The molecular orbital (MO) model provides a better explanation of chemical and physical properties than the valence bond (VB) model. – Atomic Orbital: Probability of finding the electron within a given region of space in an atom. – Molecular Orbital: Probability of finding the electron within a given region of space in a molecule. 19

Molecular Orbital Theory • Additive combination of orbitals ( ) is lower in energy than two isolated 1 s orbitals and is called a bonding molecular orbital. 20

Molecular Orbital Theory • Subtractive combination of orbitals ( *) is higher in energy than two isolated 1 s orbitals and is called an antibonding molecular orbital. 21

Molecular Orbital Theory • Molecular Orbital Diagram for H 2: 22

Molecular Orbital Theory • Molecular Orbital Diagrams for H 2– and He 2: 23

Molecular Orbital Theory • Additive and subtractive combination of p orbitals leads to the formation of both sigma and pi orbitals. 24

Molecular Orbital Theory • Second-Row MO Energy Level Diagrams: 25

Molecular Orbital Theory • MO Diagrams Can Predict Magnetic Properties: 26

Molecular Orbital Theory • Bond Order is the number of electron pairs shared between atoms. • Bond Order is obtained by subtracting the number of antibonding electrons from the number of bonding electrons and dividing by 2. BO = Bonding electrons – antibonding electrons 2 27

Example 2: Molecular Orbital Theory • The B 2 and C 2 molecules have MO diagrams similar to N 2. What MOs are occupied in B 2 and C 2, and what is the bond order in each? Would any of these be paramagnetic? 28

* B 2 2 p *2 p B has 5 electrons So B 2 has 10 elec 2 p 2 p Core electrons don’t count toward BO *2 s 2 s *1 s 1 s 29

*2 p 2 p C 2 Carbon has 6 electrons so C 2 has 12 electrons 2 p *2 s 2 s *1 s 1 s 30

Dipole moment • Based on electronegativity differences between atoms in a molecule • The most electronegative atom is partially negative • The less electronegative atom is partially positive • The dipole moment is the average of all dipoles in the molecule • Exception (C-H bonds do not have a dipole) 31

• If a molecule has a dipole moment it is polar • A compound could contain polar bonds but the molecule could be non-polar because there is no dipole moment • Bond dipoles cancel each other out 32

Example 3: Dipole moment • Draw the dipole moment for the following molecules, are they polar? HCl NH 3 CHCl 3 H 2 O SF 6 CCl 4 CO 2 CH 2 Cl 2 33