Chapter 9 Section 9.2

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Section 9.2 Molecular Shape and Polarity
The Shapes of Molecules
 The shape of a molecule and the polarity of its bonds
determine whether the molecule as a whole is polar.
 The shape is important in predicting properties and
how the molecule reacts with other molecules
The Shapes of Molecules
Models help us visualize the 3D structures of molecules.
Lewis Dot Structure
Review groups – what is the number we want?
Draw the Lewis dot structures of the following:
CH4
NH3
CO2
H2CO
 Double Bond- a bond formed by sharing two pairs of
electrons between two atoms
 Triple Bond- a bond formed by sharing three pairs of
electrons between two atoms
Shapes
 Linear
 Trigonal Planar
 Trigonal Planar
 Bent
 Tetrahedral
 Tetrahedral
 Trigonal Pyramidal
 Bent
Linear Shape (2 electron groups)
Trigonal Shape (3 electron groups)
Trigonal Planar
Bent
Tetrahedral (4 electron groups)
Tetrahedral
Bent
Trigonal
Pyramid
Lone pair electrons
Distort the shape of molecules
 Example: Tetrahedral geometry
No lone pairs
CH4
One lone pair
NH3
Two lone pairs
H2O
Steps in Creating a 3-D model
1) Draw Lewis structure
2) Count the # of bonds and lone pair electrons on the
central atom
3) Select geometric category
4) Place electrons and atoms that lead to most stable
arrangement ( minimize e-repulsions)
5) Determine 3-D
Some Models
Create the following models (pg 323)
HCl
CH4
NH3
H2O
C2H6
Polar and Non-Polar Molecules
Polarity is important, just like shape, in affecting the
properties of compounds. They act TOGETHER.
Polar and Non-Polar
Polar molecule – MOLECULE has a positive and
negative pole. (Different ENs) - Dipole
Non-polar molecule– MOLECULE share electrons
equally. (Same ENs)
Build the model and predict the
polarity of the following:
 CCl4
 NH3
 HF
 C2H4

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