VSEPR and Molecular Geometry 12 Nov.notebookmastmedical.org/ourpages/auto/2013/11/15/43296463... ·...
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VSEPR and Molecular Geometry 12 Nov.notebook
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November 12, 2013
VSEPR Theory
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VSEPR Theory
Nov 28:34 PM
VSEPR Theory
Valence Shell Electron Pair Repulsion
According to VSEPR theory, the molecules will adopt a shape/geometry so as to reduce the repulsion between the bonded electrons.
http://phet.colorado.edu/en/simulation/moleculeshapes
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The VSEPR number of a molecule is a three digit number that can be used to determine a molecule's shape.
Here's how you find it:
1. Draw the Lewis structure for the molecule. Locate the central atom, if applicable.
2. The first digit of the VSEPR number is the total number of electrondomains around the central atom.
VSEPR Numbers
Electron domains are either shared pairs of electronsor lone pairs of electrons
Multiple bonds (i.e. double or triple bonds) count as only ONE electron domain.
Oct 222:45 PM
3. The second digit of the VSEPR number is the total number of bondingdomains around the central atom.
4. The third digit of the VSEPR number is the total number of lone pairs around the central atom.
5. Check your work the first digit is equal to the sum of the second and third.
VSEPR Numbers (cont)
Bonding domains are single, double or triple bonds.
Each pair of electrons that are not involved in bondscounts as one lone pair.
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1 How many electron domains does CH4 have?
A 1B 2C 3D 4E 5
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2 How many electron domains does H2O have?
A 1B 2C 3D 4E 5
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H H O
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3 How many electron domains does CO2 have?
A 1B 2C 3D 4E 5
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C OO
Oct 2310:27 AM
C
NCl
F
OSB
P
I
H
Si
SeXe
CH4Draw a Lewis Structure and use that to determine the VSEPR number
H
H
HC
H
Check to make sure that each atom has a full outer shell. Now calculate the VSEPR #.
Electron domains = 4Bonding domains = 4Lone pairs of electrons = 0
Its VSEPR number is 4 4 0
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Oct 2310:27 AM
C
NCl
F
OSB
P
I
H
Si
SeXe
NF3Draw a Lewis Structure and use that to determine the VSEPR number
N F
F
F
Check to make sure that each atom has a full outer shell. Now calculate the VSEPR #.
Electron domains = 4Bonding domains = 3Lone pairs of electrons = 1
Its VSEPR number is 4 3 1
Slide for Answer
Oct 2310:27 AM
C
NCl
F
OSB
P
I
H
Si
SeXe
SiF4Draw a Lewis Structure and use that to determine the VSEPR number
F
Si
F
F F
Check to make sure that each atom has a full outer shell. Now calculate the VSEPR #.
Electron domains = 4Bonding domains = 4Lone pairs of electrons = 0
Its VSEPR number is 4 4 0
Slide for Answer
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Oct 2310:27 AM
C
NCl
F
OSB
P
I
H
Si
SeXe
PO43Draw a Lewis Structure and use that to determine the VSEPR number
O
P
O
O O
Check to make sure that each atom has a full outer shell. Now calculate the VSEPR #.
Electron domains = 4Bonding domains = 4Lone pairs of electrons = 0
Its VSEPR number is 4 4 0
Slide for Answer
Oct 2310:27 AM
FC
NCl
F
OSB
P
I
H
Si
SeXe
IF5 Draw a Lewis Structure and use that to determine the VSEPR number
FI
FF F
Check to make sure that each atom has a full outer shell. Now calculate the VSEPR #.
Electron domains = 6Bonding domains = 5Lone pairs of electrons = 1
Its VSEPR number is 6 5 1
Slide for Answer
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Molecular Geometry
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Molecular Geometry
Oct 109:13 PM
VSEPR and molecule shape prediction
According to VSEPR theory, the repulsion between electron pairs causes molecular shapes to adjust so that the valenceelectron pairs stay as far apart as possible.
The shape of a molecule plays an important role in determining its chemical and physical properties.
To determine a molecule's shape, i.e. its molecular geometry, we must first determine its electrondomain geometry.
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Oct 222:45 PM
Recall:
Electron domains are either shared pairs of electrons or lone pairs of electrons
Bonding domains are single, double or triple bonds.
Each pair of electrons that are not involved in bonds counts as one lone pair.
To determine the electrondomain geometry, look at the first number and use the following chart...
How does VSEPR theory help predict the shapes of molecules?
Oct 222:47 PM
Electron Domain Geometry
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ElectronDomain Geometry (EDG)
The EDG (2,3,4,5,or 6) gives us the general shape of the molecule, as shown here.
However, these domains do not have to be bonds.
The molecular geometry tells us if there is a bond or lone pair of electrons present, thereby specializing the general shape.
Let's take a closer look...
Oct 258:11 PM
Linear ElectronDomain Geometry
Linear
Two atoms around a central one will form a linear shape with bond angles of 180o
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Linear Molecular Geometry
There is only one molecular geometry for linear electrondomain: linear molecular geometry (220).
Oct 258:12 PM
Trigonal Planar ElectronDomain Geometry
trigonal planar
Three atoms around a central one will form a trigonal planar shape with bond angles of 120o
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Trigonal Planar Molecular Geometry
There are two molecular geometries:• Trigonal planar, if all the electron domains are bonding (330)
• Bent, if one of the domains is a nonbonding pair (321)
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120
trigonal planar(330)
117
bent(321)
Trigonal Planar Molecular Geometry
It is very important to note that unbonded pairs of electrons repel more strongly than bonded electrons thereby shrinking the bond
angle between atoms
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Tetrahedral ElectronDomain Geometry
Four atoms around a central one will form a tetrahedral shape with bond angles of 109.5o
tetrahedral
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Tetrahedral Molecular Geometry
There are three molecular geometries:Tetrahedral, if all are bonding pairs (440)
Trigonal pyramidal, if one is a nonbonding pair (431)Bent, if there are two nonbonding pairs (422)
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Tetrahedral Molecular Geometry
tetrahedral (440)
trigonal pyramidal(431)
bent (422)
109.5
107104.5
Again, note the decrease in bond angle as the number of high repelling unbonded pairs of electrons increase.
Oct 258:13 PM
Five atoms around a central one will form a trigonal bipyramidal shape with bond angles of 120o and 90o
trigonal bipyramidal
Trigonal Bipyramidal ElectronDomain Geometry
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Trigonal Bipyramidal Molecular Geometry
Trigonal bipyramidal
Seesaw
Tshaped
Linear
Oct 258:14 PM
Trigonal Bipyramidal
(550)SeeSaw (541)
TShape (532)
Linear (523)
Trigonal Bipyramidal Molecular Geometry
There are four molecular geometries for the trigonal bipyramidal electron domain geometry:
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Six atoms around a central one will form an octahedral shape with bond angles of 90o
octahedral
Octahedral ElectronDomain Geometry
Oct 258:14 PM
Octahedral Molecular Geometry
Square Planar
Octahedral
Square Pyramidal
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Octahedral (660)
Square Pyramidal
(651)
Square Planar (642)
Octahedral Molecular Geometry
There are only three molecular geometries for the octahedral electron domain geometry:
Oct 222:53 PM
VSEPR and molecular geometry
Using VSEPR numbers, you can determine molecular geometry.
VSEPR numbers are a set of 3 numbers.
1) the total number of electron domains2) the number of bonding domains*3) the number of unshared pairs of electrons
Electrondomain geometry has the same name as the first shape.
(*Remember that multiple bonds count as ONE domain)
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Oct 117:53 PM
Draw the Lewis structure for ammonia, NH3.
What are the VSEPR numbers for NH3? 4,3,1
What is the electrondomain geometry of NH3? tetrahedral
What is the molecular shape of NH3?
What is the NH bond angle in the molecule? 107
What is the formal charge on the N atom? 55 = 0
VSEPR Numbers and Molecular Geometries
triangular pyramidal
slide for answer
Oct 117:53 PM
Draw the Lewis structure for ClF3.
What are the VSEPR numbers for ClF3? 5,3,2
What is the electrondomain geometry of ClF3? trigonal bipyramidal
What is the molecular shape of ClF3? T
What would be the ClF bond angle(s)? 180, 90
What would be the formal charge on Cl? 77 = 0
VSEPR Numbers and Molecular Geometries
slide for answer
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Oct 244:59 PM
4 The methane molecule (CH4) has which geometry?
A linear
B trigonal bipyramidal
C trigonal planar
D tetrahedral
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5 Give the VSEPR number for this molecule.[*]
answ
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6 Give the VSEPR number for this molecule.
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7 Give the VSEPR number for this molecule.
F Xe F
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Oct 244:55 PM
8 Which compound below contains an atom that is surrounded by more than an octet of electrons?
A PF5
B CH4
C NBr3D OF2
[*]
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Dec 13:44 PM
9 Which of the following molecules would have a bent shape?
A SO2
B SO3
C CH4
D C2H2
E HF
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A
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Dec 13:46 PM
10 Which of the following molecules would have a 104.5 degree bond angle between atoms?
A H2SB CF3ClC CO2
D PCl3E NO3
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Dec 13:48 PM
11 The molecular shape and geometry of the nitrate ion (NO3) would be:
A bentB linearC trigonal planarD trigonal bipyramidalE tetrahedral
answ
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C