Atomic Orbitals Discussion

Things you should know when you leave Discussion today  Atomic orbitals (s, p, d, f) vs. molecular orbitals (σ, σ*, NB, π, π*, πnb)  Total Number of MO =Total Number of AO  Constructive and destructive interference (in phase and outof-phase interaction) o Sigma bond is achieved by head–on–overlap o Bonding MO (σ, π) - Constructive interference in-phase interaction o Antibonding MO (σ*, π*) - Destructive interference out-of-phase interaction o π formed from side-by-side overlap of available p AO  π* is out-of-phase overlap of available p AO  π is in-phase side-by-side overlap of available p AO  Factors that affect the formation of MOs from AOs: S.O.E. o Symmetry AOs must have a compatible orientation to achieve an overlap. o Overlap: valence orbitals of correct symmetry. o Energy: If the pair of AO's has the correct symmetry and greatest overlap, and closest in energy 1 1. If you have two atoms that together have 5 atomic orbitals, when those atoms combine to form a molecule, how many molecular orbitals are you going to have? 2. For each of the AO combinations below, draw the resulting MO. What do we call that resulting MO? a. Discuss any axes of symmetry the MO may have. b. Is this MO destructive or constructive interference (or neither or both)? c. (at home) Draw as many other combinations of AOs that give σ and σ* MOs as you can, using only s and p AOs. Discuss the relative energies of the resultant MOs you just drew. d. (at home) Draw as many other combinations of AOs that give π and π* MOs. 3. Draw correlation diagram for He2+. Bond Order (BO): B. O. = [ Hint: 1sσ* is more unstable than 1sσ is stable. (#of e− on the bonding orbitals)−(#of e− on the antibonding orbitals) a. What is the bond order of 2 He2+? ] b. Put in the order of increasing bond length H2; H2+ and He2+. Hint: find the bond orders for all the molecules or molecular ions. 4. (at home) Draw the correlation diagram for the HeH molecule. Which molecule or molecular ion will have the shortest bond HeH3+,HeH2+, HeH+, HeH, HeH-? 2 5. Consider the ionization energies for H, Na, and F. Assume z is along the bond axis. a. Draw the correlation diagram for NaF and HF: HF NaF Element H F F Na 1s 2s 2pz 3s IE (eV) 13.6 25.0 17.4 5.1 b. Discuss the relative ionic and covalent character of NaF verses HF. 6. Generally, the valence electrons on different atoms, rather than the core electrons, interact with one another. Choose the best explanation: a. Adjacent valence electron waves have the greatest overlap. b. Adjacent valence electron waves have the same energy. c. Core electron clouds have the wrong symmetry. d. Core electron clouds move with the wrong frequency. 7. When is the interaction of two AO orbitals most favored? 3  Below: General MO Correlation Diagram for second row in the periodic table. Note: This diagram is correct for O and F; for correlation diagrams involving B, C, and N, the σ and π bonding orbitals are inverted 4 8. Draw two AO energy diagrams for the atoms in CN- one atom on the left and one atom on the right, leaving space in the middle. Draw a picture of each AO. Draw the MO energy diagram and pictures of the resulting MOs in between and fill them with the electrons for the molecule. a. Draw the MO diagram Hint: count how many valence electrons and AOs you start with. b. Calculate the bond order. c. Is CN- diamagnetic or paramagnetic? d. Are the electrons centered more on C or on the N? e. If an electron is removed, will this make the bond longer or shorter? 9. (At home on a separate piece of paper) Draw two AO energy diagrams for the following atoms in [C2, N2, O2, O2−,F2, HF, BF, HB+, HO+], one atom on the left and one atom on the right, leaving space in the middle. Draw a picture of each AO. Draw the MO energy diagram and pictures of the resulting MOs in between and fill them with the electrons for the molecule. a. Calculate the bond order for each molecule. b. Discuss which of the molecular orbitals are responsible for single, double bonds, etc. c. Discuss the relative length, reactivity and ‘strength’ of these bonds. d. What are the magnetic properties of these molecules? (Which are diamagnetic and which are paramagnetic.) 5 10. Below on the left is an empty molecular orbital diagram of F2. Below to the right are various molecular orbitals, with the inter-nuclear axes shown with a dotted line. Fill in the boxes next to the molecular orbitals in part (a). Fill in the circles on the molecular orbital diagram in part (b). Carefully follow the instructions below. A B 2p 2p C D B 2s 2s F2 Fluorine E F a. In the box to the right of the molecular orbitals formed in F2 above, provide the proper name of each molecular orbital (e.g. σ2s, π*, etc). b. Match each molecular orbital on the right to the molecular orbital diagram on the left. Show your answer by placing the letter in the circle beside the molecular orbital. Letter “B” is filled in for you. c. In the molecular orbital diagram above, populate both the atomic and molecular orbitals with electrons by putting arrows on the horizontal lines for F2 and the fluorine atoms. All three systems are in the ground state. d. Using the diagram above to guide you, calculate the bond orders for the following species. F2+ : _____________ F22- : _____________ e. Naturally Fluorine is a diamagnetic molecule. Let say in the planet far, far away we found a fluorine that is paramagnetic: proposed and draw the molecular orbitals that Fluorine must have to be paramagnetic. 11. (at home) C2 is naturally is diamagnetic and O2 is paramagnetic. Propose and draw the molecular orbitals that will make C2 paramagnetic and O2 diamagnetic. 6 Handout Answers: 1. 2. 3. 5MOs N a. BO = 0.5 b. bond length: H2< H2+ < He2+ BO: 1 0.5 0.5 4. HeH+ molecule: HeH3+,HeH2+, HeH+, HeH, HeHBO: 0 0.5 1 0.5 0 5. a. draw correlation diagrams b. NaF-ionic; HF-covalent 6. A 7. SEO 8. a. 3 b. diamagnetic c. N d. Longer(BO=2.5) 9. 10. d. F2+ BO=1.5; F22- BO=0 e. Switch the order of π* and σ*2p2p D C 2p E B σ* 2s2s π* B 2s 2s F2 σ2s 2p F A A Fluorine 7 D σ* 2p2p E π F σ 2p2p