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BIO-0013 (103)


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Derek Mc Lachlin

Wednesday, Sept. 4, 2013 Chemistry of Water and Carbon BIO13L - JF, MM, KM Overview of today’s topics: (Chapter 2: sections 2.2, 2.3, 2.5) Bioskills 6&8 in Appendix 1. Atomic Structure: atomic number 2. Electron Orbitals, Chemical Reactivity, and Bond Angles 3. Covalent and Ionic Bonds 4. Polarity of Water, Acids and Bases Atomic number (number of electrons) defines reactivity Electrons occupy orbitals (2 e in every orbital), and orbitals are grouped into electron shells: Shell 1: 1 s orbital (2 e - Shell 2: 1 s orbital and 3 p orbitals (8 e Shell 3: 1 s orbital, 3 p orbitals and 5 d orbitals (18 e The lower the shell number, the closer to the nucleus and the lower the energy. 1 Chemical reactivity is largely driven by the fact that atoms are most stable when they have a filled outer shell of electrons (the noble gas configuration). The number of unpaired electrons in the outer shell is the valence. •Cmiiteic shells through electron sharing. •Cbtsrlgir: – Nonpolar covalent bonds have equal sharing – Polar covalent bonds have unequal sharing •isretfr COVALENT BONDS (sharing of electrons) The electrons spend some of their time around each atom giving each H atom the stable 2e filled orbital. 2 Each single line represents two shared electrons Carbon with 4 single bonds is tetrahedral because the s and 3p orbitals hybridize to create 4 hybrid orbitals of equal energy for the second shell. Five e- in outer shell. Six e- in outer shell Will form three covalent bonds and Will form two has a lone pair of covalent bonds and has two filled unshared electrons that can pick up a free orbitals that can proton (H ) from work as H-bond solution or act as an H- acceptors bond acceptor The geometry of bonds is dictated by the configuration of orbitals, including those that do not participate in the bonds. 3 Polar bonds occur because the two atoms in the bond have different electronegativity. H –2.2 C –2.5 N –3.0 O –3.5 F –4.0 P –2.2 S –2.5 Cl –3.0 Electronegativity is a function of valence and shielding of the valence electrons by shells of inner electrons. Electronegativity of O = 3.5; electronegativity of H = 2.2 4 Practice question Which molecule has the bonds which are most polar ? 1. H 2O 2. H 2 3. CH 4 4. NH 3 Properties of Water
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