1. Bonding & Shapes of Molecules

1. Bonding & Shapes of Molecules - 2010, Department...

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© 2010, Department of Chemistry, The University of Western Ontario 1.1 1. Bonding & Shapes of Molecules (text 1.1 – 1.7) Most of the material in this section is from first-year chem. It is being reviewed because a thorough understanding of it is crucial to the understanding of organic chemistry. A. Electronic Structure of Atoms The bonding behaviour of atoms depends entirely on electron configuration, revealed by an atom’s location on the Organic Chemist’s Periodic Table . 1 2 . 2 1 1 H 1 0 . 1 3 1 Li 3 0 . 2 5 3 B 4 5 . 2 6 4 C 3 0 . 3 7 5 N 2 5 . 3 8 6 O 1 0 . 4 9 7 F 1 9 . 0 11 1 Na 2 2 . 1 12 2 Mg 3 5 . 1 13 3 Al 4 8 . 1 14 4 Si 3 1 . 2 15 5 P 2 5 . 2 16 6 S 1 1 . 3 17 7 Cl 1 8 . 0 19 1 K 1 8 . 2 35 7 Br 1 5 . 2 53 7 I Br 35 72 . 8 1 Atomic number = # of electrons Group number = # of outer-shell electrons Electronegativity Valence = # of unpaired electrons in valence shell
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1.2 Recall: four quantum numbers used to identify electrons 1. Principal Quantum Number (n) where n = any integer greater than zero (e.g. 1, 2, 3, 4…). As the level n increases, so does the energy of the electron and its distance from the nucleus. Within each n level are sublevels designated . 2. Orbital Quantum Number ( ) where = any integer between 0 and (n – 1). For example, if n = 2, then = 0 or 1. determines the shape of the electron cloud, named: = 0 s electrons = 1 p electrons = 2 d electrons = 3 f electrons 3. Magnetic Quantum Number ( m ) where m = any integer between – through 0 to + . Thus, if = 1 ( p electron), then m = –1, 0, or +1. m represents the electron’s magnetic moment 4. Spin Quantum Number ( m s ) where m s = +½ or –½ regardless of other quantum numbers. When electrons come together, their spins may be the same (both + or both –) or opposite (one + and one –). These are respectively called parallel spins or opposite spins , and are denoted by arrows pointing up or down. (parallel, not allowed) (opposite, allowed)
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1.3 Ground state electron configuration o The lowest energy state for electrons in an atom is determined by placing electrons in pairs into atomic orbitals. o 3 d and 4 s are close in energy. o This filling-order gives the correct electronic configuration for all atoms normally seen in organic chemistry. o E.g. 17 Cl = o The valence shell electron structure, called the Lewis structure , can be easily drawn for the first 18 elements of the periodic table (highest configuration 1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 ). Valence Shell Electron Configuration IA IIA IIIA IVA VA VIA VIIA VIIIA H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar o For elements in groups IIA, IIIA, and IVA, the paired electrons become unpaired before forming bonds, so the valence is the total number of valence electrons ( e.g. C = 4, Mg = 2, etc.).
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1.4 B. Ionic, Covalent, and Polar Bonding Electronegativity is an atom’s ability to attract a pair of electrons to itself. Knowing the general trend is sufficient.
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1. Bonding & Shapes of Molecules - 2010, Department...

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