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CHE2060 Topic 1: Atoms, orbitals & bonding

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1 CHE2060 Topic 1: Atoms, orbitals & bonding
Atoms, Orbitals & Bonding Topics: Very quick history of chemistry… What is organic chemistry? Atomic models: nuclear to quantum All about orbitals How orbitals fill: electron configuration Basic bonding: valence electrons & molecular orbitals Lewis dot structures of molecules Electronegativity & bond polarity Resonance: a critical concept Orbital hybridization: key to carbon’s “flexibility” sp3 sp2 sp Free electron pairs & radicals VSEPR: classifying molecular geometry & orbital hybridization Daley & Daley, Chapter 1 Atoms, Orbitals & Bonds

2 CHE2060 Topic 1: Atoms, orbitals & bonding
VSEPR: a system for classifying molecular geometry & orbital hybridization

3 VSEPR categorizes geometry & hybridization
# lone e- pr # bonds e- pair geometry Molecular geometry Bond Angle (°) 2 linear 180 3 trigonal planar 120 1 bent < 120 4 tetrahedral 109.5 trigonal pyramidal < 109.5 <109.5 5 trigonal bipyramidal 90, 120, 180 seesaw T-shaped 90, 180 6 octahedral square pyramidal square planar sp sp2 sp3 sp3d sp3d2

4 sp2 geometries Molecules whose central atom is sp2 hybrized have similar electron pair & molecular geometries Electron pair geometry: trigonal planar Molecular geometry: trigonal planar (zero lone pair) bent (1 lone pair) A closer look at molecular geometries: Note that each lp produces a different geometry; When there is more than one lp, note that they are found as far away from each other as possible. Generally, switching any two substituents (to form an isomer) pushes some groups closer together and creates a less stable molecule.

5 sp3 geometries Molecules whose central atom is sp3 hybrized have greater differences between electron pair & molecular geometries Electron pair geometry: tetrahedral Molecular geometry: tetrahedral (zero lone pairs) trigonal pyramidal (1 lone pairs) Bent (2 lone pairs)

6 sp3d geometries Molecules whose central atom is sp3d hybrized have more differences between electron pair & molecular geometries Electron pair geometry: trigonal bipyramidal Molecular geometry: trigonal bipyramidal (zero lone pairs) seesaw (1 lone pairs) T-shaped (2 lone pairs) linear (3 lone pairs)

7 sp3d2 geometries Molecules whose central atom is sp3d2 hybrized have a bit less variation between electron pair & molecular geometries Electron pair geometry: octahedral Molecular geometry: octahedral (zero lone pairs) square pyramidal (1 lone pairs) square planar (2 lone pairs)

8 Exercise: VSEPR geometries & hybridizations
32 ve- :Cl: C Cl: :Cl .. Central C has 4 bonding e- pairs Central C has no lone e- pairs Therefore, tetrahedral geometry Try carbon tetrachloride, CCl4 8 ve- N H .. Central N has 3 bonding e- pairs Central N has 1 lone e- pair Therefore, tetrahedral geometry that looks like trigonal planar. Try ammonia, NH3 Try beryllium fluoride, BeF2 Be F: :F 16 ve- .. Central Be has 2 bonding e- pairs Central Be has 0 lone e- pairs So geometry is linear Try boron trifluoride, BF3 24 ve- B :F: F: :F .. Central B has 3 bonding e- pairs Central B has 0 lone e- pairs So geometry is trigonal planar

9 Exercise: put it together
Ignore non-valence electrons. For each molecule shown below, how many: orbitals bonding orbitals non-bonding orbitals antibonding orbitals how many are occupied? …and likely geometry? 1s + 1s + 1s + 2s + 2p (3) = 7 4 filled orbitals (3 bonding; 1 free pair) 3 empty antibonding orbitals H N: 1s + 1s + 3s + 3p (3) = 6 4 filled orbitals (2 bonding; 2 free pairs) 2 empty antibonding orbitals S .. H NH3 H2S HCl CO2 CH3OH CH3CH3 1s + 3s + 3p (3) = 5 4 filled orbitals (1 bonding; 3 free pr) 1 empty antibonding orbital Cl: .. H .. 2s + 2p (3) + 2s + 2p (3) = 8 8 filled orb. (4 bonding; 4 free pr) C O 1s + 1s + 1s + 1s + 2s + 2p (3) + 2s + 2p (3) = 12 7 filled orbitals (5 bonding; 2 free pairs) 5 empty antibonding orbitals H C O H .. 1s (x6) + 2s + 2p (3) + 2s + 2p (3) = 14 7 filled orbitals (7 bonding; 0 free pairs) 7 empty antibonding orbitals C C H H

10 For more VSEPR diagrams & examples:
.. Great set of diagrams from the University of Pretoria, South Africa Uses the ‘AXE’ code where: A is the central atom X is a bond to the central atom (can be single, double or triple) E is a free or lone pair of electrons

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