Transcript Molecular Geometry: Multiple Centers, Exceptions, and Polarity

Multiple Bonds, Multiple Centers, Exceptions,
and Polarity
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Objective
 Today I will be able to:
 Summarize the exceptions and inner workings of
molecular geometries
 Draw Lewis Structures to represent the valance electrons
of atoms
 Predict the molecular shape of a molecule using the
VSEPR theory
Evaluation/ Assessment
 Informal assessment – Listening to group interactions as
they complete the illustrating ionic and covalent compounds
practice and the molecular shapes lab.
 Formal Assessment – Analyzing student responses to the
exit ticket and the practice worksheets.
Common Core Connection
 Make sense of problem and persevere in solving them
 Reason abstractly and quantitatively
 Use appropriate tools strategically
 Look for and make use of structure
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Warm – Up
Explain: Exceptions and Polarity notes
Explore and elaborate: Molecular Shapes
Lab
◦ Informal Assessment
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Evaluate: Exit Ticket
• Formal assessment
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Today I will be able to:
◦ Summarize the exceptions and inner workings of
molecular geometries
◦ Draw Lewis Structures to represent the valance
electrons of atoms
◦ Predict the molecular shape of a molecule
using the VSEPR theory
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STEM Fair
◦ In Class Presentations Wednesday January 22
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Warm-Up
Exceptions & Polarity Notes
Molecular Shapes Lab
Exit Ticket
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Which shape has the largest bond angle?
How many lone pairs and bonds formed are
in the trigonal pyramidal shape?
Draw the molecular shape for phosphorus
pentachloride.
Sample Problem 1
PROBLEM:
Writing Lewis Structures for Molecules
with One Central Atom
Write a Lewis structure for CCl2F2
Sample Problem 2
PROBLEM:
Writing Lewis Structures for Molecules
with More than One Central Atom
Write the Lewis structure for methanol (molecular
formula CH4O), an important industrial alcohol that
is being used as a gasoline alternative in car
engines.
Multiple Bonds
If there are not enough electrons for the central atom to attain an
octet, a multiple bond is present.
Step 5: If the central atom does not have a full octet,
change a lone pair on a surrounding atom into another
bonding pair to the central atom, thus forming a multiple
bond.
Sample Problem 3
PROBLEM:
Writing Lewis Structures for Molecules
with Multiple Bonds
Write Lewis structures for the following:
(a) Ethylene (C2H4), the most important reactant in
the manufacture of polymers
(b) Nitrogen (N2), the most abundant atmospheric
gas
Exceptions to the Octet Rule
Molecules with Electron-Deficient Atoms
B and Be are
commonly electrondeficient.
Odd-Electron Species
A molecule with an odd number of electrons is
called a free radical.
Exceptions to the Octet Rule
Expanded Valence Shells
An expanded valence shell is only possible for nonmetals from Period 3
or higher because these elements have available d orbitals.
Sample Problem 4
PROBLEM:
Writing Lewis Structures for Octet-Rule
Exceptions
Write a Lewis structure and identify the octet-rule
exception for (a) SClF5; (b) BFCl2.
How to draw and decipher
Figure 10.12 The tetrahedral shapes around the central atoms
and the overall shapes of ethane (A) and ethanol
(B).Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ethane
ethanol
CH3CH3
CH3CH2OH
Sample Problem 5
PROBLEM:
Predicting Molecular Shapes with More
Than One Central Atom
Determine the shape around each of the central
atoms in acetone, (CH3)2CO.
Molecular Shape and Molecular Polarity
Overall molecular polarity depends on both shape and bond
polarity.
The polarity of a molecule is measured by its dipole moment
(μ), which is given in the unit debye (D).
A molecule is polar if
- it contains one or more polar bonds and
- the individual bond dipoles do not cancel.
The orientation of polar molecules in an electric field.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Molecules are randomly
oriented.
Molecules become oriented
when the field is turned on.
Bond Polarity, Bond Angle, and Dipole Moment
Example: CO2
The DEN between C (EN = 2.5) and O (EN = 3.5) makes each C=O
bond polar.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
CO2 is linear, the bond angle is 180°, and the individual bond
polarities therefore cancel. The molecule has no net dipole
moment (μ = 0 D).
Bond Polarity, Bond Angle, and Dipole Moment
Example: H2O
The DEN between H (EN = 2.1) and O (EN = 3.5) makes each H-O
bond polar.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
H2O has a V shaped geometry and the individual bond polarities do
not cancel. This molecule has an overall molecular polarity. The O
is partially negative while the H atoms are partially positive.
Bond Polarity, Bond Angle, and Dipole Moment
Molecules with the same shape may have different polarities.
CCl4
Bonds are polar, but
individual bond polarities
cancel.
CHCl3
Bond polarities do not cancel.
This molecule is polar (μ = 1.01
D).
Sample Problem 7
Predicting the Polarity of Molecules
PROBLEM:
For each of the following use the molecular shape
and EN values and trends to predict the direction
of bond and molecular polarity, if present.
(a) Ammonia, NH3
(b) Boron trifluoride, BF3
(c) Carbonyl sulfide, COS (atom sequence
SCO)
Finish up and complete the polarity section
now!
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Determine the shape
CH3Cl
CH2O
BeCl2
BCl3
PF5
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SF6
XeF4
SF4
ICl3
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O2K+
ClO2-1
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CN-1
CO32SO42-
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Draw the Lewis Structure for CO2
Determine the shape and bond angles of CO2
according to the VSEPR theory.