Chapter 9 Notes AP CHEMISTRY Galster Molecular Geometry

Chapter 9 Notes AP CHEMISTRY Galster

Molecular Geometry A. VESPR Theory A. Abbreviation for valence shell electron pair repulsion. B. Valence electrons will arrange themselves to reduce repulsion i. e. they will be as far apart as possible 1. 2 e-sets 1. Bond angle: 2. Shape name: 3. AZx designation: 2. 3 e- sets 1. Bond angle: 2. Shape name: 3. AZx designation:

1. 4 e-sets 1. Bond angle: 2. Shape name: 3. AZx designation: 2. 5 e- sets 1. Bond angle: 2. Shape name: 3. AZx designation: 3. 6 e-sets 1. Bond angle: 2. Shape name: 3. AZx designation:

Electron Set Geometry Name Molecular Geometry Name Example CO 2 AZ 2 (2 edomains) Linear AX 2 Linear BF 3 AZ 3 (3 edomains) Trigonal planar AX 3 Trigonal planar SO 2 AZ 3 (3 edomains) Trigonal planar AX 2 E Bent 3 -D Shape

Electron Set Geometry AZ 4 (4 edomains) Name Molecular Geometry Name Example CH 4 Tetrahedral AX 4 Tetrahedral NH 3 Tetrahedral AX 3 E Trigonal pyramidal H 2 O Tetrahedral AX 2 E 2 Bent HCl Tetrahedral AXE 3 Linear 3 -D Shape

Electron Set Geometry AZ 5 (5 edomains) Name Molecular Geometry Name Example PCl 5 Trigonal bipyramidal AX 5 Trigonal bipyramidal SF 4 Trigonal bipyramidal AX 4 E See-Saw Cl. F 3 AZ 5 (5 edomains) Trigonal bipyramidal AX 3 E 2 T-shaped Xe. F 2 AX 2 E 3 Linear 3 -D Shape

Electron Set Geometry Name Molecular Geometry Name Example SF 6 AZ 6 (6 edomains) Octahedral AX 6 Octahedral Br. F 5 AZ 6 (6 edomains) Octahedral AX 5 E Square pyramidal Xe. F 4 AZ 6 (6 edomains) Octahedral AX 4 E 2 Square planar 3 -D Shape

Effect of non-bonding electrons on bond angles A. Order of Repulsion 1. Nonbonding-nonbonding > nonbonding-bonding > bonding-bonding B. Trends A. Angle between nonbonding pairs increase B. Angles between bonding pairs decrease C. Examples 1. CH 4 tetrahedral 2. NH 3 tetrahedral trig. Pyramidal 3 Water, H 2 O tetrahedral bent

• 4. Sulfur tetrafluoride, SF 4 • • 5. COCl 2

Polarity of Molecules A. Review of polar bonds 1. Polar bonds - ∆EN is less than or equal to 0. 5 (but >1. 7, ionic) 2. Nonpolar bonds ∆EN is less than 0. 5 B. Molecular polarity 1. Nonpolar bonds a. b. c. d. Diatomic molecules or others with nonpolar bonds Ex: H 2, Cl 2, I 2 No dipole moment So… molecules with all nonpolar bonds are nonpolar 2. Cancellation of dipoles a. b. c. d. Depends on geometry “tug of war” Polar bonds – yes Dipole moment – no Ex: CCl 4, CO 2

Examples of Nonpolar Molecules • CO 2 • Si. Cl 5 • As. Cl 5 • ICl 4 -1

Examples of Polar Molecules • Polar bonds: yes • Dipole Moment? Yes • H 2 O SF 4 Br. F 5

Which of these are polar? 1. 2. 3. 4. 5. 6. SO 2 I 3 -1 CHCl 3 PCl 3 Xe. Cl 4 Se. Br 4

Atomic Orbitals: Hybridization • Atomic Orbitals – covalent bonds result when orbitals in valence level overlap • **VESPR theory says that the number of unpaired electrons tells you the number of bonds the atom will form. • Ex: H 2 • • HCl • Cl 2

Hybrid Orbitals • Mixing of orbitals • Pure orbitals cannot always be used to explain bonding 1. Basics a. Atomic orbitals are combined to create new hybrid orbitals b. The number of hybrid orbitals formed is equal to the number of atomic orbitals combined * We are not increasing the number of orbitals, just rearranging them. a. The names of the hybrid orbitals are based on the type and number of atomic orbitals combined. b. The hybrid orbitals are degenerate and have the same shape.

Process of hybridization • A. methane, CH 4 • B. BF 3 • C. Be. Cl 2

• D. H 2 O • E. NH 3 • F. Xe. F 4

Hybridization and Electron Set Geometry • • • sp = one s & one p = AZ 2 linear sp 2= one s & two p = AZ 3 trigonal planar sp 3= one s & three p = AZ 4 tetrahedral sp 3 d = one s, three p, and one d = AZ 5 trigonal bipyramidal sp 3 d 2 = one s, three p, and two d = AZ 6 octahedral

Multiple Bonds • Multiple bonds do not affect the electron set geometry of a molecule • Multiple bonds make attractions (i. e. bond) stronger and shorter • Types of bonds • Single bond = σ (sigma) bond • Double bond = σ (sigma) bond – first bond π (pi) bond – 2 nd or subsequent bonds Ex: C 2 H 4

• Triple bond • Ex: C 2 H 2 (Acetylene)

• Sigma bonds are characterized by • Head-to-head overlap • Cylindrical symmetry of electron density about the internuclear axis • Pi bonds are characterized by • Side-to-side overlap • Electron density above and below the internuclear axis

Delocalized Electrons: Resonance • In reality, each of the four atoms in the nitrate ion has a p orbital. • The p orbitals on all three oxygens overlap with the p orbital on the central nitrogen. • This means the electrons are not localized between the nitrogen and one of the oxygens, but rather are delocalized throughout the ion.

Resonance • The organic molecule benzene has six σ bonds and a p orbital on each carbon atom. • In reality the π electrons in benzene are not localized but delocalized • The even distribution of the π electrons in benzene makes the molecule unusually stable.

Molecular-Orbital (MO) Theory • Through valence bond theory effectively conveys most observed properties of ions and molecules, there are some concepts better represented by molecular orbitals. • In MO theory, we invoke the wave nature of electrons • If waves interact constructively the resulting orbital is lower in energy: a bond

Molecular-Orbital (MO) Theory • If waves interact destructively, the resulting orbital is higher in energy: an antibonding molecular orbital.

MO Theory • In H 2 the two electrons go into the bonding molecular orbital • The bond order is one half the difference between the number of bonding and antibonding electrons

MO Theory • For hydrogen, with two electrons in the bonding MO and none in the antibonding MO, the bond order is ½ (2 – 0) = 1

MO Theory • In the case of He 2, the bond order would be ½ (2 - 2) = 0 • Therefore, He 2 does not exist.

MO Theory • For atoms with both s and p, there are two types of interactions: • The s and the p orbitals that face each other overlap in σ fashion. • The other two sets of p orbitals overlap in π fashion.

MO Theory • The resulting MO diagram looks like this : • There are both σ and π bonding molecular orbitals and σ* and π* antibonding molecular orbitals

MO Theory • The smaller p-block elements in the second period have a sizable interaction between the s and p orbitals. • This flips the order of the σ and π molecular orbitals in these elements

Second-Row MO Diagrams