ANNOUNCEMENTS If you have questions about your exam 2 grade, write to me or Chem200@sdsu.edu. Chapter 7 homework due Nov, 9 th. Chapter 8 homework due Nov. 16 th. Exam 3 is 11/17 at 2 pm.
LECTURE OBJECTIVES Chapter 8.4 Molecular Orbital Theory Describe bonding in terms of molecular orbital (MO) theory. Use MO theory to determine bonding/bond order. Complete MO diagrams for selected diatomic molecules. Determine magnetic properties of diatomic molecules using MO theory/diagrams. Chapter 9 Gases Describe the unique properties of gases and the ideal gas law
LECTURE OBJECTIVES Chapter 8.4 Describe bonding in terms of molecular orbital theory (MO theory). Use MO theory to determine bonding/bond order. Complete MO diagrams for selected diatomic molecules. Determine magnetic properties of diatomic molecules using MO theory/diagrams.
CENTRAL THEMES OF MOLECULAR ORBITAL THEORY A molecule is viewed on a quantum mechanical level as a collection of nuclei surrounded by delocalized molecular orbitals. Molecular wave functions are the summation of the atomic wave functions. A bonding molecular orbital (MO) is formed by wave functions that reinforce each other. This yields a region of high electron density between the nuclei. Wave functions that cancel each other are antibonding molecular orbitals. A node of zero electron density occurs between the nuclei when this happens.
BONDING & ANTIBONDING WAVE FUNCTIONS Bonding Amplitude of the two waves combined is higher than each individually. Antibonding Amplitude of the two waves combined cancels each wave to zero at a point.
Bonding Antibonding High e - density between nuclei Lower energy & more stable than the individual atomic orbitals that formed the MO. Low e - density between nuclei Higher energy & less stable than the individual atomic orbitals that formed the MO.
ENERGIES OF THE BONDING & ANTIBONDING MO S IN H 2 Bonding MO is lower in energy than the individual atomic orbitals Antibonding MO is higher in energy than the individual atomic orbitals
MOLECULAR ORBITAL DIAGRAM The filling of Energy 1s σ*1s 1s molecular orbitals follows the same concepts as were used to fill atomic orbitals. σ1s AO MO AO of H of H2 of H
BOND ORDER MO theory can be used to calculate the bond order. Bond Order = # of e- in bonding MOs - # of e- in antibonding MOs 2 bond order = ½(2-0) =1 single bond bond order = ½(2-2) =0 no bond
QUESTION Using MO theory, determine which option below is correct pertaining to the potential Code existence of He2 and He2 +. He2 may exist He2 + may exist A Yes Yes B No No Energy 1s σ*1s 1s C Yes No D No Yes σ1s AO MO AO
MO DIAGRAMS OF HE 2 & HE 2 + Energy 1s σ*1s 1s Energy 1s σ*1s 1s σ1s σ1s AO of He MO of He2 + AO of He + AO of He MO of He2 AO of He He2 + bond order = 1/2 He2 bond order = 0
BONDING IN S-BLOCK HOMONUCLEAR DIATOMIC MOLECULES Energy 2s σ*1s 2s Energy 2s σ*1s 2s Li2 σ1s Be2 σ1s Li2 bond order = 1 Be2 bond order = 0 https://www.youtube.com/watch?v=pehe1zv5bps
CONTOURS & ENERGIES OF SIGMA & PI MO FORM COMBINATIONS OF 2P ATOMIC ORBITALS all p orbitals The pz orbital acts like the py orbital, forming a π2p MO.
RELATIVE MO ENERGY LEVELS FOR PERIOD 2 HOMONUCLEAR DIATOMIC MOLECULES MO energy levels for O2, F2 & Ne2. No 2s-2p mixing With 2s-2p mixing MO energy levels for B2, C2 & N2. Energy Mixing of 2s and 2p orbitals lowers the energy of σ2s & σ*2s, but raises energy of σ2p & σ*2p.
MO MIXING Orbital energies decrease across the period as the effective nuclear charge increases, and the atomic radius decreases.
QUESTION Predict the bond order for O2 and O2 +. Answer: 2 and 1.5 A 2 and 2 B 2 and 2.5 C 2 and 3 D None of the above E Energy
QUESTION Predict the bond order for N2 and N2 +. Answer: 3 and 2 A 3 and 2.5 B 3 and 3 C 3 and 3.5 D None of the above E Energy
MO occupancy and molecular properties for B2 through Ne2. Higher bond order results in greater bond energy and shorter bond length. The pairing of the electrons correlates with the magnetic properties.
Diamagnetic substance MAGNETISM Substance whose electrons are all paired ( ) Weakly repelled by magnetic fields. Paramagentic substance Substance with one or more unpaired electrons ( ) Attracted by magnetic fields.
QUESTION Which of the following species will be paramagnetic? C2, C2 +, N2 C2 C2 + N2 All are paramagnetic None are paramagnetic Answer: A B C D E Energy
MOLECULES WITH PI BONDS The energy difference between the highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO) of molecules with multiple π-bonds is often in the UV or visible energy range. Lycopene in tomatoes has a small enough HOMO-LUMO gap that all visible light can be absorbed except red light. The more π-bonds involved in resonance structures, the lower the energy difference of the HOMO-LUMO gap. Lycopene
SUNBLOCK/SUNSCREEN Sunscreens are designed with HOMO-LUMO gaps that will allow the molecules to absorb UV light, but not visible light. OH H 2 N O para-aminobenzoic acid (PABA) Visible Light UV Light
CHAPTER 9 - GASES