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Resonance and VSEPR Models

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Kasey Hickey

on 26 February 2013

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Transcript of Resonance and VSEPR Models

8.12
Resonance Resonance occurs when more
than one Lewis dot structure
accurately represents a molecule Resonance Structures: The positioning in bonds is not set
While atoms must stay in place, bonds and pairs of electrons can move between like elements
The movement of the location of bonds creates different resonance structures Consider that 2 Lewis Dot Structures accurately represent the ion H C O H C O O -1 -1 The arrows show that the average of these structures represent the actual structure Draw the Lewis Dot Structure for
(CHO2)-1 Hints and Reminders: There will be more than one correct Lewis Dot Structure
Atoms must stay in place
The ion is charged
Electron pairs and bonds can only move between atoms of the same element How can we tell which resonance form is the best? We use FORMAL CHARGE! Formal charge is a comparison between the atom’s valence electrons and those that belong to it in the molecule Formal charge= (Group # of the element)-(# of bonds it is involved in in the molecule)- (# of lone pair electrons the atom has in the molecule) The resonance structure with the charge closest to zero is the best
Negative charges are best when they are on the most electronegative elements draw two resonance structure of HN3 including formal charge Let's look at an example... PRACTICE! *Start by drawing two of the resonance structures possible* The following structures are possible: Now find the atoms' formal charges ... Remember FC= (group#)-(#of bonds)-(#of electrons) H N N N H N N N H N N N H N N N H N N N FC=1-1-0=0 FC=5-2-4=-1 FC=5-4=+1 FC=5-3-2=0 0 +1 -1 0 H N N N FC=1-1-0=0 FC=5-3-2=0 FC=5-4=+1 FC=5-2-4=-1 0 0 -1 +1 Now let's look at VSEPR models..... **Note** Consider: Few molecules form from nonmetals with odd numbers of electrons
i.e. NO
These molecules do not work with the localized electron model
Localized e- model works with pairs of e-
A more complex model is needed to resemble these structures (CHO2)-1 is an ion taken from a formic acid. Did you know it is produced in large amounts in liver cells, the mitochondria of embryonic cells, and in cancer cells? 8.13 VSEPR MODELS V
S
E
P
R
Models alence
hell
lectron
air
epulsion What Do They Do? Minimize the repulsions between electron pairs in a molecule
Predict the 3-dimensional molecular shape How? VESPR models are based off of an atom's number of occupied areas Occupied Areas Single Bonds

Double Bonds

Triple Bonds

Electron Pairs *each is one occupied area* 2 Occupied Areas 2 Bonds
Shape:Linear
Bond angle: 180 degrees Ex: BeI2 3 Occupied Areas: Bonds: 3
Lone Pairs: 0
Shape: Trigonal Planar
Bond Angle: 120 degrees Ex: BF3 180 120 4 Occupied Areas: Bonds: 4
Lone Pairs: 0
Shape: Tetrahedral
Bond Angle: 109.5 degrees Ex: CF4 109.5 3 Occupied Areas: Bonds: 2
Lone Pairs: 1
Shape: Bent
Bond Angle: 120 degrees Ex: (NO2)- 120 4 Occupied Areas: Bonds: 3
Lone Pairs: 1
Shape: Trigonal Pyramidal
Bond Angle: 107 degrees Ex: PCl3 107 4 Occupied Areas: Bonds: 2
Lone Pairs: 2
Shape: Bent
Bond Angle: 105 degrees Ex: H2O 105 When there are 5 occupied areas, there are two types of axes
Axial axis goes up and down
Equatorial axes go side to side and front and back
It is best for e- to be in equatorial positions 5 Occupied Areas: Bonds: 5
Lone Pairs: 0
Shape: Trigonal Bipyramidal
Bond Angles:
90 degrees (axial to equatorial)
120 degrees (equatorial) Ex: CF4 5 Occupied Areas: Bonds: 3
Lone Pairs: 2
Shape: T-shape
Bond Angles:
90 degrees (axial to equatorial)
180 degrees (equatorial) Ex: ClF3 5 Occupied Areas: Bonds: 4
Lone Pairs: 1
Shape: See-saw
Bond Angles:
90 degrees (axial to equatorial)
120 degrees (equatorial) Ex: TeCl4 5 Occupied Areas: Bonds: 2
Lone Pairs: 3
Shape: Linear
Bond Angles 180 degrees Ex: (I3)- 90 120 90 180 180 Molecules with 6 occupied areas have no axial or equatorial positions 6 Occupied areas Bonds: 6
Electron Pairs: 0
Shape: Octahedral
Bond Angle: 90 Ex: (PF6)- 90 6 Occupied areas Bonds: 5
Electron Pairs: 1
Shape: Square Pyramidal
Bond Angle: 90 degrees Ex: (SbCl5)-2 90 6 Occupied areas Bonds: 4
Electron Pairs: 2
Shape: Square Planar
Bond Angle: 90 degrees Ex: (ICl4)- 90 Predicting VSEPR Models: Start by drawing the Lewis do structure for the molecule (any resonance structure can be used)
Count the correct number of e- pairs and arrange them to minimize repulsions (maximum distance apart)
Determine the position of the atoms
Use the occupied areas, positions of atoms, and numbers of bonds and electrons to predict molecular structure VSEPR Models: For molecules with no central atom, the structure will be a combination of the previous models

For molecules with a central atom, any resonance structure can be used PRACTICE: Determine the shape and bond angles of the following molecules 1. (IO4)- 2. (CH3)+ 4. (CH3)- 3. BrF3 Answers: 1. (IO4)- 2. (CH3)+ 4. (CH3)- 3. BrF3 Tetrahedral (109.5) Trigonal Planar (120) T-Shaped (90, 180) Trigonal Pyramidal
(90, 120) O Thanks for bonding with Us! all images used in this prezi are taken from: http://liakatas.org/chemblog/wp-content/uploads/2008/07/vsepr.png 8.12 Resonance
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