1. Introduction c
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Created by:
christopherdale on January 5, 2011
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Geometry of molecules, polarity of covalent molecules, electronegativity
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Terms | Definitions |
|---|---|
 Tetrahedral molecules | Tetrahedral methane CH4 has 109.5 degree bond angles and it is drawn with two solid lines for the bonds that are in the plane of the paper, a dashed line for the bond that goes behind the plane of the paper, and a wedge-shaped bond that goes in front of the plane of the paper. H2O has a 104.5 degree bond angle because nonbonding electrons take up space around the oxygen atom. |
| Planar molecules | Ethylene is a flat molecule CH2=CH2, with C=C bond length of about 134 pm. |
| Linear molecules | Acetylene's triple bond has length 120 pm. |
| Pancake bond | Two electron, four center bond observed, for example, in some polycyclic aromatic hydrocarbon dimers. See for example: http://pubs.acs.org/doi/abs/10.1021/ja058387z |
| Polar covalent bonds | The covalent bond between two atoms that are the same is nonpolar, but when there are two different atoms covalently bonded together, the electrons of the bond are drawn closer to the more electronegative atom. A polar covalent bond has unequal sharing of the electrons of the bond and it is shown by writing a partial negative or partial positive (δ+ or δ-) over the atom. Also referred to as a bond dipole. You can also draw a symbol sorta like this: ┼──→ |
| Electronegativity | Concept developed by Linus Pauling. In general, electronegativities increase moving up in a group or to the right in a period of the periodic table. Bonds between metals at the far left of the table and nonmetals at the far right are so polar that they are called ionic. A difference of greater than about 2.0 it is considered ionic (e.g. NaCl has difference of 2.23). C-H bond does not have much polarity. The electronegativity of C is 2.5 and that of H is 2.1. The electronegativity of oxygen is 3.5, making the O-H bond very polar. Chlorine has electronegativity 3.0. See this chart: http://en.wikipedia.org/wiki/Electronegativity |
| Dipole moment, μ | A separation of electric charge by a finite distance. In CO2 and CCl4 the sum of the bond dipoles is equal to zero and the dipole moment of the entire molecule is zero. Water, μ = 1.84D CH3Cl, μ = 1.86D |
| Factors affecting electronegativity | -the number of protons in the nucleus -the distance from the nucleus -the amount of screening by inner electrons |
| Electronegativities Table | Element Allen Pauling Allred Mulliken H 2.3 2.2 2.2 3.059 He 4.16 ? 5.5 ? Period 2 Li 0.912 0.98 0.97 1.282 Be 1.576 1.57 1.47 1.987 B 2.051 2.04 2.01 1.828 C 2.544 2.55 2.5 2.671 N 3.066 3.04 3.07 3.083 O 3.61 3.44 3.5 3.215 F 4.193 3.98 4.1 4.438 Ne 4.787 ? 4.84 4.597 Period 3 Na 0.869 0.93 1.01 1.212 Mg 1.293 1.31 1.23 1.63 Al 1.613 1.61 1.47 1.373 Si 1.916 1.9 1.74 2.033 P 2.253 2.19 2.06 2.394 S 2.589 2.58 2.44 2.651 Cl 2.869 3.16 2.83 3.535 Ar 3.242 ? 3.2 3.359 Period 4 K 0.734 0.82 0.91 1.032 Ca 1.034 1 1.04 1.303 Ga 1.756 1.81 1.82 1.343 Ge 1.994 2.01 2.02 1.949 As 2.211 2.18 2.2 2.256 Se 2.424 2.55 2.48 2.509 Br 2.685 2.96 2.74 3.236 Kr 2.966 3 2.94 2.984 Period 5 Rb 0.706 0.82 0.89 0.994 Sr 0.963 0.95 0.99 1.214 In 1.656 1.78 1.49 1.298 Sn 1.824 1.96 1.72 1.833 Sb 1.984 2.05 1.82 2.061 Te 2.158 2.1 2.01 2.341 I 2.359 2.66 2.21 2.88 Xe 2.582 2.6 2.4 2.586 |
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