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Unformatted text preview: Chem 105 Nielson Recitation 10/11/07
Molecular shape Polar and nonpolar molecules 1. Complete all of the following steps for each compound below. a. Draw the Lewis structure, including all nonzero formal charges. b. Determine the electron-region geometry. c. Determine the molecular shape (or ion shape). d. Estimate the values of the bond angles. If a bond angle is distorted from what would
be predicted by the shape, then indicate if it is > or < than that value. Lewis
structure NF3 ° .F»§~Et
. 2F: 02 ‘
“ ‘ 14
CHC13 :‘Cl ’Q’gl.
do" -" ‘ Ob /‘.‘ \ 0.. '
' g E. «l N
C1F3 a a .CrF.‘
.O. .l ..
6 av Electron-region geometry tetrahedral linear tetrahedral tetrahedral octahedral octahedral trigonal bipyramidal trigonal
planar tetrahedral trigonal planar Molecular
planar bent bent Angles AFN}? < need 3 atoms
for bond angle LHCCI =1 LQSQ < LFSF = 90 LFXeF = 90 LFCIF = 90 LOCOI = 120 4000 < 109.5 LQNQ < 120 2. The electron-region geometry can also be determined around each atom in larger
molecules. The geometry is still based on the model of VSEPR (Valence Shell Electron
Pair Repulsion). A shape can also be determined around each atom, but it is no longer
the molecular shape, because the molecule is much larger than that atom and the atoms to
which it is bonded. Determine the electron-region geometry and shape around each C, N,
and/or 0 in the following molecules (You drew these Lewis structures on the last
recitation sheet). CH3NH2 C = tetrahedral, tetrahedral N = tetrahedral, trigonal pyramidal CH3CN C = tetrahedral, tetrahedral C = linear, linear CH3CH20H both C = tetrahedral, tetrahedral, O = tetrahedral, bent 3. Once you know the concepts of bond dipoles, Lewis structures, and molecular shape,
you can determine whether a molecule is polar or nonpolar. Polar molecules have a dipole moment (remember sec. 8.4) where the electron density is
unevenly distributed over a molecule, because of electronegativity differences between
atoms. Polar molecules must have polar bonds. Remember bending the water stream in
lecture with a balloon that I had charged by rubbing on my head? Water is polar. Nonpolar molecules do not have net uneven distribution of electron density in the
molecule. Some nonpolar molecules have nonpolar bonds and some have polar bonds
that are oriented so that they cancel out each other’s bond dipoles. If the molecule has symmetry and the central atom is surrounded by identical atoms and
m lone pairs, then the molecule is nonpolar, even if it has polar bonds. So linear ABZ,
trigonal planar AB3, tetrahedral AB4, trigonal bipyramidal AB5, and octahedral AB6 are
all nonpolar molecules. (Can you see why this is true?) Are the molecules below polar or nonpolar? BF3 nonpolar NF3 polar
CHCl3 polar CCl4 nonpolar
SOClz polar SF6 nonpolar
XeF4 nonpolar (XeF bonds all C1F3 polar cancel each other out.) ...
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