Chapter 4 Chemical Bonding: Molecular Geometry and Hybridization PDF

Chapter 4 Chemical Bonding: Molecular Geometry and Hybridization of Atomic Orbitals Prof. Nasser Y. Mostafa Chemistry Department – Faculty of Science Suez Canal University 1 Geometry of the molecules ??? H2 H2O CH4 H-H Why molecule...

Chapter 4 Chemical Bonding: Molecular Geometry and Hybridization of Atomic Orbitals Prof. Nasser Y. Mostafa Chemistry Department – Faculty of Science Suez Canal University 1 Geometry of the molecules ??? H2 H2O CH4 H-H Why molecules take these geometry ? 2  Geometry of the molecules ??? CO2 NH3 DNA Understanding the bonding of C, N, and O will 3  Molecular LEGO The main LEGO units in organic chemistry are C, O, N and H  Valence Bond Theory As two H atoms approach each other, their 1s orbitals begin to interact and each electron begins to feel the attraction of the other proton. Gradually, the electron density builds up in the region between the two nuclei (red color). Eventually, a stable H molecule is formed. 2 Change in electron density as two hydrogen atoms approach each other. 5 6  Bonding in Carbon Electron Configurations C: 1s 2 2s2 2p2 6 Hybridization – mixing of two or more atomic orbitals to form a new set of hybrid orbitals. Mix at least 2 nonequivalent atomic orbitals (e.g. s and p). Hybrid orbitals have very different shape from original atomic orbitals. Number of hybrid orbitals is equal to number of pure atomic orbitals used in the hybridization process. Covalent bonds are formed by: Overlap of hybrid orbitals with atomic orbitals Overlap of hybrid orbitals with other hybrid orbitals 8 sp3 Hybridization of Carbon Formation of sp3 Hybrid Orbitals Formation of Covalent Bonds in CH4 sp2 Hybridization of Carbon Formation of sp2 Hybrid Orbitals Unhybridized 2pz orbital (gray), which is perpendicular to the plane of the hybrid (green) orbitals.  Bonding in Ethylene, C2H4 Sigma bond () – electron density between the 2 atoms Pi bond () – electron density above and below plane of nuclei of the bonding atoms Another View of  Bonding in Ethylene, C2H4 sp Hybridization of Carbon Bonding in Acetylene, C2H2 Another View of the Bonding in Ethylene, C2H2 CH4 NH3 H2O 20 Describe the bonding in CH2O. H C O H C – 3 bonded atoms, 0 lone pairs C – sp2 21  Valence Bond Theory and NH3 N – 1s22s22p3 3 H – 1s1 If the bonds form from overlap of 3 2p orbitals on nitrogen with the 1s orbital on each hydrogen atom, what would the molecular geometry of NH3 be? If use the 3 2p orbitals predict 90o Actual H-N-H bond angle is 107.3o 23  Sigma () and Pi Bonds () Single bond 1 sigma bond Double bond 1 sigma bond and 1 pi bond Triple bond 1 sigma bond and 2 pi bonds How many  and  bonds are in the acetic acid (vinegar) molecule CH3COOH? O H  bonds = 6 + 1 = 7 H C C O H  bonds = 1 H 24 Molecular Orbital (MO) Configurations 1. The number of molecular orbitals (MOs) formed is always equal to the number of atomic orbitals combined. 2. The more stable the bonding MO, the less stable the corresponding antibonding MO. 3. The filling of MOs proceeds from low to high energies. 4. Each MO can accommodate up to two electrons. 5. Use Hund’s rule when adding electrons to MOs of the same energy. 6. The number of electrons in the MOs is equal to the sum of all the electrons on the bonding atoms. 25 Delocalized molecular orbitals are not confined between two adjacent bonding atoms, but actually extend over three or more atoms. Example: Benzene, C6H6 Delocalized  orbitals 26 Electron density above and below the plane of the benzene molecule. H2 H2O CH4  Chemistry In Action: different carbon structures Buckyball Graphite Carbon nanaotube