Experiment 15: Atomic Orbitals, Bond Length, and Molecular Orbitals

Transcription

1 Experiment 15: Atomic Orbitals, Bond Length, and Molecular Orbitals Introduction Molecular orbitals result from the mixing of atomic orbitals that overlap during the bonding process allowing the delocalization of electrons involved in bonding. When bonding occurs, the electron density (negative charge) between two nuclei (positive charge) increases enough to balances the repulsive force between the two nuclei. When the attractive and repulsive forces balance, the energy is at a minimum and the atoms are at their natural bond length. The atomic and molecular orbitals have interesting visual representations. To see them you need to be familiar with the visualization process. Purpose The goals of this experiment are: Become familiar with the relations between quantum numbers and atomic orbital shapes. To create hybrid orbitals by adding and subtracting atomic orbitals. To visualize bond formation To determine bond lengths To visualize molecular orbitals of various types involved in the bonding process. Lab Procedure Part 1: Using Quantum Numbers to Generate Atomic Orbitals. 1. Maple is a computer program capable of computing and visually representing complex equations such as Schrodinger's electron wave functions which are the mathematical descriptions of orbitals. Start the Maple worksheet titled OrbitalMaker.sws located in the Chem1a folder on the computer near you. 2. Follow all directions in the worksheet which will generate plots of the electron density of orbitals. Part II : Finding Normal Bond length for H 2 He 2 and Your own Molecule using ChemViz ChemViz is a simulation program what is capable of calculation and generating images of atoms, molecules, atomic orbitals and electron densities. We will look at the output of these simulations to determine the bond length and stability of three diatomic molecules. a. The files you will be working with are at: (Note: There is no www in the address.) b. You will begin by looking at H 2. Select Find lowest energy and bond length for H 2. On the following page you will see the animation of bringing the two H atoms closer together looking from three different perspectives- along the X, Y, and Z axis. You can look at each frame of the animation by Clicking the Animation Frames link. The top frame is the view of two hydrogen atoms 4.0 Angstroms (Å) apart. Each following frame corresponds to the H 2 atoms being 0.2 A closer (3.8 Å, 3.6 Å etc) till they are only 0.2 Å apart. Again the bond formation

2 is pictured along the X, Y, and Z axis. Click the Back button on your browser to return to the previous menu. c. View the output of the graph the energy vs. distance relationship by selecting "Energy Graph" to determine the approximate lowest energy and therefore the approximate bond length. 4. Repeat for He 2, and a third molecule of your choosing (O 2, N 2 or F 2 ). Questions 1. Describe the images you saw. Sketch the Energy vs. Distance graph in your lab book accurately with labeled axis. 2. When atoms that can bond approach one another, why does the energy decrease, reach a minimum, and then increase? How is this different from atoms that don't form bonds? 3. Would it make a difference if you looked at it from a different angle? Why or why not? Hint: Consider the symmetry of the orbitals. 4. What is the normal bond length for H 2 and the molecule you selected in terms of distance in angstroms?

3 Part III: Looking at the Molecular Orbitals This time we will use ChemViz Waltz to examine animations of molecular orbitals (MO s) starting with the lowest energy MO as the first frame and the highest energy MO as the last frame. a. Return to the main menu at b. Click on View step animations of molecular orbitals for N 2, O 2 or F 2 The animation will step through views of 10 molecular orbitals for the molecule you selected starting with the lowest in energy and ending with the molecular orbital highest in energy. The views again are along the X, Y, and Z axis. You will find it easier to view them one at a time by clicking STOP at the bottom, then RESET to return to the first MO then STEP to look at the second MO and so on. Be sure to check the frame numbers under the X, Y and Z views to make sure they are all the same. If you click on a molecule in View electron densities for molecular orbitals, then click on animate frames in the lower left corner, all of the frames are shown on one page. Questions 5. Sketch the orbital electron densities in your notebook. Identify each molecular orbital as 1s, 1s 2s, 2s etc. Use your textbook for help. 6. Next to the images you have sketched, describe the atomic and molecular orbital designations i.e. A 1s + a 1s orbital gives a 1s, molecular orbital. 7. Indicate which ones represent bonding, and which ones represent anti-bonding? How can you tell? (Look the electron density between atoms for each orbital by looking at the Z-slice of the Step Animation to make this determination.) 8. Draw an energy diagram showing the atomic and molecular orbitals for your molecule and match the energy levels to the molecular orbitals you imaged. Sketch each orbital next to the corresponding molecular energy level with an appropriate 'bonding' or 'anti -bonding' designation next to each i.e. 1s, 1s 2s, 2s etc. You will need your book for this. 9. Were the molecular orbitals in the 'correct' order? Why or why not? 10. Why are the relative orbital sizes of the 1s and 2s different? 11. How do bonding and antibonding orbitals differ?

4 Name _ Pre-lab 1. Consider each of the following sets of quantum numbers (n, l, ml, s). Decide if each set is valid or not valid. For valid sets, identify the orbital the set describes (i.e. 2p). For sets that are not valid, give an explanation as to why the set is not valid. a) n = 2, l = 1, ml = 0, s = ½ b) n = 0, l = 0, ml = 0, s = ½ c) n = 3, l = 2, ml = 2, s = ½ d) n = 3, l = 2, ml = 3, s = ½ 2. Explain the difference between Schrödinger s orbitals and Bohr s electron orbits. 3. For each quantum number, list the symbol and give a brief description. Quantum # Symbol Description Principal Angular Momentum Magnetic Spin 4. Complete the following table to indicate the total number of orbitals in each energy level (n). In the remaining columns, specify how many of those orbitals are s, p, d, and f. Level n Total orbitals s-orbitals p-orbitals d-orbitals f-orbitals

5 5. Draw appropriate MO diagrams for the diatomic molecules Si 2 and SO +. For each, determine the bond order and whether the molecule is paramagnetic or diamagnetic. Si 2 AO MO AO 3p * 3p * 3p 3p 3p 3p 3s * 3s 3s _ 3s SO + AO MO AO 3p * 3p * 2p 3p 3p 3p 2s 3s * 3s _ 3s