# E absolute value of the charge of an e 1602 x 10 19 c

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e = absolute value of the charge of an e - (1.602 X 10 -19 C) Calculate U(r) for Na + and Cl - . NaCl has a bond length (r) = 2.36Å. 5

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U(r) = ( )( )( ) = 4 π (8.854 x 10 -12 C 2 J -1 m -1 )( ) Convert to kJ/mol U(r) = -9.774 x 10 -19 J x _________ x _______________ = Simple ionic model predicts: E = - E d = - 444 kJ/mol Experiments measure: E = - E d = - 411 kJ/mol The discrepancy results from the following approximations: ignored repulsive interactions. Result:_________ E d than experimental value. treated Na + and Cl as ________________________. ignored quantum mechanics. This simple model is applicable only to very ionic bonds. III. POLAR COVALENT BONDS Perfectly-ionic and perfectly-covalent bonds are the two extremes of bonding. In reality, most bonds fall somewhere in the middle. A polar covalent bond is an ____________ sharing of electrons between two atoms with different electronegativities ( χ ). Consider H-Cl versus H-H (Pauling electronegativity values are given): H – Cl χ H = 2.2 χ Cl = 3.2 H δ + – Cl δ where δ is fraction of a full charge (e) that is asymmetrically distributed. H – H H 2 is a “perfectly” covalent bond, δ = 0. 6
Dipole moment Asymmetric charge distribution results in an electric dipole, two unlike charges separated by a finite distance. G We can quantify charge separation by defining a dipole moment, µ . C m δ+ δ− Dipole moment is measured in C m or in debye μ = Q r r Magnitude of charge 1 debye = 1 D = 3.336 x 10 –30 C m Separation where Q = ⏐δ e μ In chemistry, the arrow points toward the negative charge in a polar bond. Polar molecules have a non-zero net dipole moment. χ H = 2.2 χ C = 2.6 χ N = 3.0 C O O CO 2 O H H H 2 O χ O = 3.4 molecule molecule In large organic molecules and in biomolecules, such as proteins, we often consider the number of polar groups within the molecule. 7 For example, let’s compare vitamin A to vitamin B9 For example, let’s compare vitamin A to vitamin B9 Which vitamin contains a higher number of polar bonds? vitamin ________ Which vitamin contains a higher number of polar bonds? vitamin ________ HO O H C O CH 3 CH C CH 3 3 3 H H H H H 2 HC C C C C C C H H C CH H H 2 2 C C 2 C N O C C C C C C O H H H H H H 2 O CH 2 HC C H H 2 2 C C C N C C C C C N C C N H H C C H H 2 CH 2 3 OH HC C C N N NH 2 Vitamin A Vitamin A Vitamin B9 (_____________________) Vitamin B9 (_____________________) ________________ soluble ________________ soluble ________________ soluble ________________ soluble H C C H C H H C C H C C H H 2 C O H 3 C CH 3 CH 3 CH 3 CH 3 H C CH HC C C H N H C N H O HC H 2 C CH 2 C OH C O HO O H 2 C C N C HC C N C N C N O NH 2 H
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• Fall '08
• Vogel
• Atom, Chemical bond

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