CHEM BC2001 General Chemistry Week 4, Class #7 PDF
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Columbia University
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These are lecture notes covering molecular orbital theory. They discuss electrons in molecules, molecular orbitals, and atomic orbitals. An example of a molecular orbital diagram for H2 is shown.
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9/25/24 CHEM BC2001 GENERAL CHEMISTRY WEEK 4, CLASS #7 WED., SEPTEMBER 25TH Announcements & Reminders Exam I is Monday, 9/30. Remember to bring a calculator with you to the exam. H...
9/25/24 CHEM BC2001 GENERAL CHEMISTRY WEEK 4, CLASS #7 WED., SEPTEMBER 25TH Announcements & Reminders Exam I is Monday, 9/30. Remember to bring a calculator with you to the exam. Homework #4 will be done using Odyssey (not Achieve). Odyssey installation instructions are posted to Canvas. The assignment will be due to Canvas by 10 PM on Monday,10/7. Student hours and TA workshops will not be held on exam days. PLEASE MAKE SURE CELL PHONES ARE SILENCED AND AWAY & ALL ELECTRONIC DEVICES ARE DISCONNECTED FROM WIFI. 9/25/24 MOLECULAR ORBITAL THEORY (CONT’D) Electrons in molecules exist in molecular orbitals (MOs). Molecular orbitals are analogous to atomic orbitals (AOs). Electrons in MOs have quantized energy. To form molecular orbitals, we add and subtract atomic orbitals. We add and subtract atomic orbitals of the correct symmetry and energy. 9/25/24 1 9/25/24 MOLECULAR ORBITAL DIAGRAM FOR H2 H H2 H (1) (1) σ1s* 1s 1s σ1s Bond Order = 1 MOLECULAR ORBITAL DIAGRAM FOR He2 He He2 ? He (2) (2) σ1s* 1s 1s σ1s Bond Order = 0 2 9/25/24 Periodic Table of the Elements 9/25/24 VALENCE ELECTRONS FOR ROW 2 ELEMENTS Recall ordering of atomic orbitals: Valence Orbitals Figure 5.11b 9/25/24 3 9/25/24 COMBINING S ORBITALS INTO MOLECULAR ORBITALS: SIGMA BONDING NODE + + + – sigma antibonding MO: out-of-phase, destructive interference, DESTABILIZING + + sigma bonding MO: + in-phase, constructive interference, STABILIZING 9/25/24 Figure 9.3 COMBINING S ORBITALS INTO MOLECULAR ORBITALS: SIGMA BONDING NODE sigma antibonding MO: + – out-of-phase, destructive interference, DESTABILIZING Phase Change sigma bonding MO: + in-phase, constructive interference, STABILIZING In-Phase Combination 9/25/24 Figure 9.3 4 9/25/24 REPRESENTATION OF ALL THREE 2P ORBITALS + – – – + + 2pz 2py 2px Every p atomic orbital has a nodal plane, and there is a phase change across this node. 9/25/24 The lower n, the closer the electron on average, to the nucleus COMBINING PZ ORBITALS Nodal plane ∗ 𝜎!"# Out-of-Phase Combination 𝜎!"# In-Phase Combination These are also sigma (s) molecular orbitals, because the orbital interaction occurs along the internuclear axis. 9/25/24 Figure 9.7 5 9/25/24 COMBINING PX AND PY ORBITALS Nodal plane π∗!"% Out-of-Phase Combination π!"% In-Phase Combination These are called pi (p) molecular orbitals, because the orbitals are 9/25/24 perpendicular to the internuclear axis. Figure 9.8 MO ENERGIES FOR ROW 2 DIATOMICS (LI2 – N2) Representative depictions of MOs E (Note: only p2py * is shown.) (Note: only p2py is shown.) + – + Figure 9/25/249.13 6 9/25/24 Periodic Table of the Elements 9/25/24 MO ENERGIES FOR ROW 2 DIATOMICS 9/25/24 Figure. 9.11 7 9/25/24 MO ENERGIES FOR ROW 2 DIATOMICS Li2 through N2 O2 through Ne2 E E 9/25/24 8
