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Compounds and Their Bonds
Shapes and Polarity of Molecules
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VSEPR In the valence-shell electron-pair repulsion theory
(VSEPR), the electron groups around a central atom are arranged as far apart from each other as possible have the least amount of repulsion of the negatively charged electrons have a geometry around the central atom that determines molecular shape
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Shapes of Molecules The three-dimensional shape of a molecule
is the result of bonded groups and lone pairs of electrons around the central atom is predicted using the VSEPR theory (valence-shell-electron-pair repulsion)
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Guide to Predicting Molecular Shape (VSEPR Theory)
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Polar Molecules A polar molecule
contains polar bonds (determined using electronegativity of the atoms in the bond) 0.4 <EN< 1.7 has a separation of positive and negative charge called a dipole, indicated with + and – has dipoles that do not cancel + – • • H–Cl H—N—H dipole H dipoles do not cancel
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Nonpolar Molecules A nonpolar molecule contains nonpolar bonds
Cl–Cl H–H or has a symmetrical arrangement of polar bonds
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Determining Molecular Polarity
Determine the polarity of the H2O molecule. Process: Determine shape of molecule using VSEPR (focus on symmetry) Determine polarity of the bonds (EN) Calculate net dipole
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Learning Check Determine the shape of each of the following
molecules and whether they are polar or nonpolar. Explain. A. PBr3 B. HBr C. Br2 D. SiBr4
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Solution A) pyramidal; dipoles don’t cancel; polar
B) linear; one polar bond (dipole); polar C) linear; nonpolar bond; nonpolar D) tetrahedral; dipoles cancel; nonpolar
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Summary A molecule will be polar if it is asymmetrical AND contains polar bonds. A molecule will be non-polar if it is asymmetrical BUT contains non-polar bonds. A molecule will be non-polar if it is symmetrical, regardless of bond type.
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What Good is This?? The polarity of a molecule will tell you a lot about properties such as solubility, boiling/melting points, etc. when you compare it to other similar molecules.
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VSEPR Theory and Polarity
Complete the worksheet from yesterday by determining if the molecules/ions are polar or non-polar. Read section 4.5 Attempt some problems from the electronic problem set. (see unit page) Complete Page 227 #1-3
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