AP Chemistry Rapid Learning Series - 15

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Chemistry :: Biology :: Physics :: Math

Rapid Learning Cen...
AP Chemistry Rapid Learning Series - 15

Learning Objectives
By completing this tutorial you will learn…
Valence Bond Theo...
AP Chemistry Rapid Learning Series - 15

Valence Bond Theory
and the Octet Rule

5/86

Definition: Valence Shell
Valence S...
AP Chemistry Rapid Learning Series - 15

Definition: Valence Bond Theory
Valence Bond Theory
– Bonds are formed
by overlap...
AP Chemistry Rapid Learning Series - 15

Determining # of
Valence Electrons

9/86

Valence Electrons and the Periodic Tabl...
AP Chemistry Rapid Learning Series - 15

Valence Electrons of Transition Metals
The transition metals don’t have easy patt...
AP Chemistry Rapid Learning Series - 15

Definition: Lewis Structure
Lewis Structure – 2D visualization of
how electrons a...
AP Chemistry Rapid Learning Series - 15

Lewis Structures
Structures—
Covalent Compounds
with 2 Elements

15/86

Binary Co...
AP Chemistry Rapid Learning Series - 15

Binary Covalent Structure - 2
Another example:

1

Arrange the atoms symmetricall...
AP Chemistry Rapid Learning Series - 15

Lewis Structures
- Multiple Bonds

19/86

Multiple Bond Example - 1a
Begin with t...
AP Chemistry Rapid Learning Series - 15

Multiple Bond Example - 1b
When the previous steps do not result in full valences...
AP Chemistry Rapid Learning Series - 15

Multiple Bond Example - 2b
When the previous steps do not result in full valences...
AP Chemistry Rapid Learning Series - 15

Lewis Structures - Covalent
Compounds with More
Than 2 Elements

25/86

What Orde...
AP Chemistry Rapid Learning Series - 15

Finishing the Lewis Structures
Once you’ve arranged the atoms, finish the process...
AP Chemistry Rapid Learning Series - 15

Example 2 - 2
1

Arrange the atoms according to the formula.

2

Determine the # ...
AP Chemistry Rapid Learning Series - 15

Moving Hydrogen Atoms Around
Sometimes, in order to have all atoms with full vale...
AP Chemistry Rapid Learning Series - 15

Definition: Polyatomic Ion
Polyatomic Ion – Group of atoms
covalently bonded that...
AP Chemistry Rapid Learning Series - 15

Polyatomic Anion
A anion is a negatively charge ion (gain of electrons).

1
2
3
4...
AP Chemistry Rapid Learning Series - 15

Another Approach - 1
1

Arrange the atoms symmetrically or according to
the chemi...
AP Chemistry Rapid Learning Series - 15

Another Approach - 3
3

Put one pair of electrons in between each set of
atoms.

...
AP Chemistry Rapid Learning Series - 15

Another Approach - 5
5

If any atoms do not have full valences, move a
lone pair ...
AP Chemistry Rapid Learning Series - 15

A Larger Compound - 2
2

Determine the total # of valence electrons for each
elem...
AP Chemistry Rapid Learning Series - 15

A Larger Compound - 4
4

Place lone pairs around the most electronegative
atom fi...
AP Chemistry Rapid Learning Series - 15

Lewis Structures Ionic Compounds

47/86

Definition: Ionic Compound
Ionic Compoun...
AP Chemistry Rapid Learning Series - 15

Ionic Compound Example
An ionic compound is between metals and non-metals.

1

De...
AP Chemistry Rapid Learning Series - 15

Exceptions to
the Octet Rule

51/86

Common Exceptions to the Octet Rule
# of Val...
AP Chemistry Rapid Learning Series - 15

Exception Examples
“Full” with 6
electrons

F B F
F

S
F
Has 10
electrons

53/86
...
AP Chemistry Rapid Learning Series - 15

VSEPR Theory

V alence
S hell
E lectron
i
P air
Repulsion Theory
55/86

What Does...
AP Chemistry Rapid Learning Series - 15

Definition: Electron & Molecular Geometry
Electron Geometry – Uses the
VSEPR theo...
AP Chemistry Rapid Learning Series - 15

Chemical Formulas for Geometry
Each geometry has a “generic” chemical formula.
Ce...
AP Chemistry Rapid Learning Series - 15

Trigonal Planar Geometry
AX3
3 electron regions

Bonds 120° apart

Named: It’s a ...
AP Chemistry Rapid Learning Series - 15

Trigonal Bipyramidal Geometry
AX5
5 electron regions

Bonds: Inside “Triangle” = ...
AP Chemistry Rapid Learning Series - 15

Determining Electron Geometry
Electron geometry is determined by electron regions...
AP Chemistry Rapid Learning Series - 15

Geometry with Lone Pairs
Use the formula to determine the electron geometry.
Each...
AP Chemistry Rapid Learning Series - 15

Bent Geometry with 2 Lone Pairs
AX2E2

Remove 2 atoms

Start with 4 electron regi...
AP Chemistry Rapid Learning Series - 15

See-Saw Geometry
AX4E

Remove 1 atom

Start with 5 electron regions
Trigonal Bipy...
AP Chemistry Rapid Learning Series - 15

Linear with 3 Lone Pairs
AX2E3

Remove 3 atoms

Start with 5 electron regions
Tri...
AP Chemistry Rapid Learning Series - 15

Square Planar Geometry
AX4E2

Remove 2 atoms

Start with 6 electron regions
Octah...
AP Chemistry Rapid Learning Series - 15

Determining Geometry with Lone Pairs
Formula

Name of geometry

AX2E

Bent

AX2E2...
AP Chemistry Rapid Learning Series - 15

Lewis
Structures &
The AP Exam

79/86

Lewis Structures in the Exam
Common Lewis ...
AP Chemistry Rapid Learning Series - 15

Multiple Choice Questions
Problems often combine bonding theory with Lewis
Struct...
AP Chemistry Rapid Learning Series - 15

Answering Free Response Questions
Drawing structures along with labeling and brie...
AP Chemistry Rapid Learning Series - 15

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You have successfully completed
the core tutorial

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Lewis structuresvsepr theory

  1. 1. AP Chemistry Rapid Learning Series - 15 Rapid Learning Center Chemistry :: Biology :: Physics :: Math Rapid Learning Center Presents … p g Teach Yourself AP Chemistry Visually in 24 Hours 1/86 http://www.RapidLearningCenter.com Lewis Structures AP Ch i t R id Learning Series Chemistry Rapid L i S i 2/86 Wayne Huang, PhD Kelly Deters, PhD Russell Dahl, PhD Elizabeth James, PhD Debbie Bilyen, M.A. © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com Rapid Learning Center www.RapidLearningCenter.com/ © Rapid Learning Inc. All rights reserved. 1
  2. 2. AP Chemistry Rapid Learning Series - 15 Learning Objectives By completing this tutorial you will learn… Valence Bond Theory The O t t R l Th Octet Rule Lewis Structures for: Elements Covalent Compounds Polyatomic Ions Ionic Compounds p Valence Shell Electron Pair Repulsion Theory Electron and Molecular Geometry 3/86 Concept Map Previous content Chemistry New content Studies Matter One type is Valance Bond Theory 1 bonding theory is Shown with Lewis Structures Compounds Molecular Geometry Used to determine Electron Geometry 4/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 2
  3. 3. AP Chemistry Rapid Learning Series - 15 Valence Bond Theory and the Octet Rule 5/86 Definition: Valence Shell Valence Shell – Outermost shell of electrons; the electrons with the highest principal energy level number; the electrons that form chemical bonds. Cl: 1s2 2s2 2p6 3s2 3p5 7 valence electrons Br: [Ar] 4s 2 3d10 4p 5 7 valence electrons 6/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 3
  4. 4. AP Chemistry Rapid Learning Series - 15 Definition: Valence Bond Theory Valence Bond Theory – Bonds are formed by overlap of valence orbitals. Valence Bond H H H H s-orbital Both atoms get to “count” the electrons that are being shared between the two. 7/86 Definition: Octet Rule Octet Rule – Most atoms are more stable with a full valence shell (which is a noble gas configuration). A full shell has 8 electrons (“oct-” = 8). More exceptions will be discussed soon, but for now, Hydrogen is an exception. y g Hydrogen’s valence shell only contains a 1s orbital, which can only hold 2 electrons. Therefore, hydrogen is most stable with 2 electrons. 8/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 4
  5. 5. AP Chemistry Rapid Learning Series - 15 Determining # of Valence Electrons 9/86 Valence Electrons and the Periodic Table The main groups of the periodic table have # of electrons = main group #. 1 2 3 4 5 6 7 8 10/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 5
  6. 6. AP Chemistry Rapid Learning Series - 15 Valence Electrons of Transition Metals The transition metals don’t have easy patterns. Here are some of the common elements: Element Valence Electrons Al 3 Zn 2 Cd 2 Ag 1 Au 3 11/86 Lewis Structures 12/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 6
  7. 7. AP Chemistry Rapid Learning Series - 15 Definition: Lewis Structure Lewis Structure – 2D visualization of how electrons are shared to form bonds between atoms. Also called: - Electron Dot Structures - Dot Structures - Lewis Dot Structures 13/86 Lewis Structures of Elements How to draw an element: 1 Use the element’s symbol to represent the nucleus ( ) and core (non-valence) electrons. 2 Determine the number of valence electrons from the position on the Periodic Table. 3 Draw the electrons around the “nucleus”—one on each side before doubling up (Hund’s Rule—place one in each orbital before doubling). Example: Draw the Lewis Structure for an oxygen atom. Oxygen is in the 6th main group. There are 6 valence electrons. O 14/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 7
  8. 8. AP Chemistry Rapid Learning Series - 15 Lewis Structures Structures— Covalent Compounds with 2 Elements 15/86 Binary Covalent Structures - 1 For compounds with only 2 different non-metals: 1 Arrange the atoms symmetrically. 2 Determine the # of valence electrons for each element. 3 Draw the valence electrons—do not double up where 2 atoms are bonding. 4 When atoms have 8 (2 for H), the structure is done. Example: Draw the Lewis Structure for CH4 H Carbon is in the 4th main group. There are 4 valence electrons. Hydrogen is in the 1st main group. There is 1 valence electron. H C H H 16/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com Carbon now has 8 electrons it’s sharing. Each hydrogen has 2 electrons it’s sharing. 8
  9. 9. AP Chemistry Rapid Learning Series - 15 Binary Covalent Structure - 2 Another example: 1 Arrange the atoms symmetrically. 2 Determine the # of valence electrons for each element. 3 Draw the valence electrons—do not double up where 2 atoms are bonding. 4 When atoms have 8 (2 for H), the structure is done. Example: Draw the Lewis Structure for NH3 Nitrogen is in the 5th main group. There are 5 valence electrons. Hydrogen is in the 1st main group. There is 1 valence electron. H N H H 17/86 Nitrogen now has 8 electrons it’s sharing. Each hydrogen has 2 electrons it’s sharing. Lone Pairs and Bonding Pairs Lone Pair Electrons not shared in a bond H N H H Bonding Pair Electrons shared between two atoms Lone Pairs are “counted” only by the one atom. y y Lone Pairs are important and must be drawn even though they aren’t bonding. Bonding Pairs are “counted” by both atoms that are sharing them. 18/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 9
  10. 10. AP Chemistry Rapid Learning Series - 15 Lewis Structures - Multiple Bonds 19/86 Multiple Bond Example - 1a Begin with the same steps: 1 Arrange the atoms symmetrically. 2 Determine the # of valence electrons for each element. 3 Draw the valence electrons—do not double up where 2 atoms are bonding. 4 When atoms have 8 (2 for H), the structure is done. Example: Draw the Lewis Structure for CO2 . Carbon is in the 4th main group. There are 4 valence electrons. Oxygen is in the 6th O C O main group. There are 6 valence electron. Currently, carbon only has 6. And each oxygen only has 7. 20/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 10
  11. 11. AP Chemistry Rapid Learning Series - 15 Multiple Bond Example - 1b When the previous steps do not result in full valences: 5 Move two unpaired electrons on adjacent atoms to g bond together. Repeat until all atoms have full valence shells. Example: Draw the Lewis Structure for CO2 . Now Carbon has 8. O C O And each oxygen also has 8. 21/86 Multiple Bond Example - 2a Begin with the same steps: 1 Arrange the atoms symmetrically. 2 Determine the # of valence electrons for each element. 3 Draw the valence electrons—do not double up where 2 atoms are bonding. 4 When atoms have 8 (2 for H), the structure is done. Example: Draw the Lewis Structure for HCN . Carbon has 4 valence electrons H C Hydrogen has 1 valence electron Nitrogen has 5 valence electrons. N But the carbon and nitrogen each only have 6 electrons. 22/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 11
  12. 12. AP Chemistry Rapid Learning Series - 15 Multiple Bond Example - 2b When the previous steps do not result in full valences: 5 Move two unpaired electrons on adjacent atoms to g bond together. Repeat until all atoms have full valence shells. Example: Draw the Lewis Structure for HCN . Now all valence shells are full. H C N 23/86 Double and Triple Bonds Double Bond O C O H C N Triple Bond A double bond is 2 pairs of electrons being shared. Double bonds are shorter and stronger than single bonds. 24/86 A triple bond is 3 pairs of electrons being shared. Triple bonds are shorter and stronger than double bonds. © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 12
  13. 13. AP Chemistry Rapid Learning Series - 15 Lewis Structures - Covalent Compounds with More Than 2 Elements 25/86 What Order do the Elements Go In? When there are more than two elements, how do you arrange them? 1 “COOH” is a carboxylic acid. 2 Hydrogen and halogens (F, Cl, Br, I) can’t go in the middle. 3 Of the elements that can go in the middle, write in the order they’re given. 4 Write the hydrogen and halogen atoms around what they’re next to in the formula. Example: H5C2OH Write in this order HH H C C O H HH 26/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 13
  14. 14. AP Chemistry Rapid Learning Series - 15 Finishing the Lewis Structures Once you’ve arranged the atoms, finish the process: 1 Arrange the atoms according to the formula. 2 Determine the # of valence electrons for each element. 3 Draw the valence electrons - do not double up where 2 atoms are bonding. 4 When atoms have 8 (2 for H), the structure is done. Example: H5C2OH Each carbon has 4 electrons Each hydrogen has 1 electron The oxygen has 6 electrons. H H H C C O H H H 27/86 Example 2 - 1 1 “COOH” is a carboxylic acid. 2 Hydrogen and halogens (F Cl Br I) can’t go in the (F, Cl, Br, can t middle. 3 Of the elements that can go in the middle, write in the order they’re given. 4 Write the hydrogen and halogen atoms around what they’re next to in the formula. Example: BrH2CCH2COOH Carboxylic acid HHO Br C C C O H HH 28/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 14
  15. 15. AP Chemistry Rapid Learning Series - 15 Example 2 - 2 1 Arrange the atoms according to the formula. 2 Determine the # of valence electrons for each element. 3 Draw the valence electrons—do not double up where 2 atoms are bonding. 4 When atoms have 8 (2 for H), the structure is done. Example: BrH2CCH2COOH Currently, some of the atoms are full… 29/86 But one carbon and one oxygen each only have 7. H H O Br C C C O H HH Example 2 - 3 5 Move two unpaired electrons on adjacent atoms to g bond together. Repeat until all atoms have full valence shells. Example: BrH2CCH2COOH All valence shells are currently full. H H O Br C C C O H HH 30/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 15
  16. 16. AP Chemistry Rapid Learning Series - 15 Moving Hydrogen Atoms Around Sometimes, in order to have all atoms with full valence shells, a hydrogen must be bonded in a different location. Example: C3H6 Each carbon has 4 electrons. Each hydrogen has 1 electron. H H H C C C H H H Two carbons do not have full valence shells. They are not adjacent—they cannot double bond. If one hydrogen is moved to another carbon… Now two carbons right next to each other have un-full shells and can double bond. 31/86 This move of the hydrogen is not prohibited by the given information (the formula C3H6). Lewis Structures Polyatomic Ions 32/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 16
  17. 17. AP Chemistry Rapid Learning Series - 15 Definition: Polyatomic Ion Polyatomic Ion – Group of atoms covalently bonded that together have a charge. e.g. NH4+, SO42- 33/86 Polyatomic Cation A cation is a positively charge ion (loss of electrons). 1 2 3 4 Arrange the atoms according to the formula. Determine the # of valence electrons for each element. Draw the valence electrons—do not double up where 2 atoms are bonding. When atoms have 8 (2 for H), the structure is done. In this case, there is no choice but to double up on a side with a bond. Example: NH41+ Nitrogen has 5 electrons Each hydrogen has 1 electron. The +1 charge means we can remove 1 electron! H H N H H +1 H’s electron is removed to result in +1 charge 34/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 17
  18. 18. AP Chemistry Rapid Learning Series - 15 Polyatomic Anion A anion is a negatively charge ion (gain of electrons). 1 2 3 4 Arrange the atoms according to the formula. Determine the # of valence electrons for each element. Draw the valence electrons—do not double up where 2 atoms are bonding. When atoms have 8 (2 for H), the structure is done. When placing electrons around the oxygen, do not place them where there is already a pair of electrons from sulfur. Example: SO42- Sulfur has 6 electrons. Each oxygen has 6 electrons. The -2 charge means we can add 2 electrons! O O S O O -2 2 electrons are added to result in the -2 charge. 35/86 Lewis Structures Another Approach 36/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 18
  19. 19. AP Chemistry Rapid Learning Series - 15 Another Approach - 1 1 Arrange the atoms symmetrically or according to the chemical formula. Example: Draw the Lewis Structure for CO2 . O C O 37/86 Another Approach - 2 2 Determine the total # of valence electrons for each element. Example: Draw the Lewis Structure for CO2 . Counting electrons: 1 Carbon = 1 × 4 = 4 electrons 2 Oxygens = 2 × 6 = 12 electrons O C O Total = 16 electrons 38/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 19
  20. 20. AP Chemistry Rapid Learning Series - 15 Another Approach - 3 3 Put one pair of electrons in between each set of atoms. Example: Draw the Lewis Structure for CO2 . Counting electrons: 1 Carbon = 1 × 4 = 4 electrons 2 Oxygens = 2 × 6 = 12 electrons O C O Total = 16 electrons 12 14 16 39/86 Another Approach - 4 4 Place lone pairs around the most electronegative atom first (closest to F on the periodic table). Stop when you run out of electrons. Example: Draw the Lewis Structure for CO2 . Counting electrons: Oxygen is more electronegative than carbon 1 Carbon = 1 × 4 = 4 electrons 2 Oxygens = 2 × 6 = 12 electrons O C O Total = 16 electrons 10 12 0 2 4 6 8 40/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 20
  21. 21. AP Chemistry Rapid Learning Series - 15 Another Approach - 5 5 If any atoms do not have full valences, move a lone pair from an adjacent atom in to form multiple bonds. Example: Draw the Lewis Structure for CO2 . Each oxygen has 8 electrons But carbon only has 4 electrons O C O 41/86 A Larger Compound - 1 1 Arrange the atoms symmetrically or according to the chemical formula. Example: BrH2CCH2COOH H H O Br C C C O H HH 42/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 21
  22. 22. AP Chemistry Rapid Learning Series - 15 A Larger Compound - 2 2 Determine the total # of valence electrons for each element. Example: BrH2CCH2COOH Counting electrons: 1 Bromine = 1 × 7 = 7 electrons 5 Hydrogens = 5 × 1 = 5 electrons 3 Carbons = 3 × 4 = 12 electrons 2 Oxygens = 2 × 6 = 12 electrons Total = 36 electrons H H O Br C C C O H HH 43/86 A Larger Compound - 3 3 Put one pair of electrons in between each set of atoms. Example: BrH2CCH2COOH Counting electrons: 1 Bromine = 1 × 7 = 7 electrons 5 Hydrogens = 5 × 1 = 5 electrons 3 Carbons = 3 × 4 = 12 electrons 2 Oxygens = 2 × 6 = 12 electrons Total = 36 electrons H H O Br C C C O H HH 44/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 16 36 22
  23. 23. AP Chemistry Rapid Learning Series - 15 A Larger Compound - 4 4 Place lone pairs around the most electronegative atom first (closest to F on the periodic table). Stop when you run out of electrons. Example: BrH2CCH2COOH Oxygen is most electronegative, followed by bromine. Counting electrons: 1 Bromine = 1 × 7 = 7 electrons 5 Hydrogens = 5 × 1 = 5 electrons 3 Carbons = 3 × 4 = 12 electrons 2 Oxygens = 2 × 6 = 12 electrons Total = 36 electrons H H O Br C C C O H HH 0 6 16 45/86 A Larger Compound - 5 5 If any atoms do not have full valences, move a lone pair from an adjacent atom in to form multiple bonds. Example: BrH2CCH2COOH All have full valences except one carbon and one oxygen. H H O Br C C C O H HH Now all have full valences. 46/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 23
  24. 24. AP Chemistry Rapid Learning Series - 15 Lewis Structures Ionic Compounds 47/86 Definition: Ionic Compound Ionic Compound – Metals transfers electrons to non metals. The non-metals. resulting ions form an electrostatic attraction. e.g. KCl, Na2SO4 48/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 24
  25. 25. AP Chemistry Rapid Learning Series - 15 Ionic Compound Example An ionic compound is between metals and non-metals. 1 Determine the # of valence electrons for each atom. 2 Draw the valence electrons. 3 Transfer electrons from the metals to the nonmetals to fill valence shells. The metal will be left with no electrons in the electrons shell “drawn”. However, the next inner shell is full and it now the “valence” shell. Example: KCl Potassium has 1 electron. Chlorine has 7 electrons. K +1 Cl -1 49/86 Polyatomic Ionic Compound Example An ionic compound is between metals and non-metals. 1 Determine the # of valence electrons for each atom. 2 Draw the valence electrons. 3 Transfer electrons from the metals to the nonmetals to fill valence shells. The total charge should = 0 Example: Na2SO4 Each sodium has 1 electron SO4 is a polyatomic ion—it must be covalently bonded first. Na Na +1 +1 -2 O -1 O S O O 50/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 25
  26. 26. AP Chemistry Rapid Learning Series - 15 Exceptions to the Octet Rule 51/86 Common Exceptions to the Octet Rule # of Valence Electrons when “Full” Element(s) 2 H, He 6 B, Be >8 Any element in the 3rd period and below Elements in period 3 and below have empty “d” orbitals that can be used to hold more than 8 valence electrons. 52/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 26
  27. 27. AP Chemistry Rapid Learning Series - 15 Exception Examples “Full” with 6 electrons F B F F S F Has 10 electrons 53/86 Electron and Molecular Geometry 54/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 27
  28. 28. AP Chemistry Rapid Learning Series - 15 VSEPR Theory V alence S hell E lectron i P air Repulsion Theory 55/86 What Does VSEPR Mean? Bonds are made of shared electrons (negatively charged subatomic particles) Negatively charged things repel each other Valence shell electron pair Bonds form as far apart from each other (and other electrons) as possible repulsion 56/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 28
  29. 29. AP Chemistry Rapid Learning Series - 15 Definition: Electron & Molecular Geometry Electron Geometry – Uses the VSEPR theory with the electron regions around the central atom. atom An electron bond is a bond (single, double or triple…they all count as one region). Molecular Geometry – Uses the VSEPR theory with the atoms bonded around the central atom. 57/86 How to Count Electron Regions Electron geometry depends on electron regions surrounding the central atom. 1 2 H N H H 4 H C 3 N 1 58/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 2 The triple bond is only one electron region. 29
  30. 30. AP Chemistry Rapid Learning Series - 15 Chemical Formulas for Geometry Each geometry has a “generic” chemical formula. Central Atom Lone Pairs A X E Atoms bonded to central atom (“ligand”) e.g. X A X AX3E X 59/86 Linear Geometry AX2 2 electron regions Bonds 180° apart Named: looks like a “line” Examples: CO2, BeH2 60/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 30
  31. 31. AP Chemistry Rapid Learning Series - 15 Trigonal Planar Geometry AX3 3 electron regions Bonds 120° apart Named: It’s a flat (“planar”) triangle. Examples: BF3, C2H4 61/86 Tetrahedron Geometry AX4 4 electron regions Bonds 109.5° apart Named: If each plane (defined by 3 points) is covered, it’s a 4 (“tetra”) sided object (“hedron” ). Examples: CH4, SO2Cl2 62/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 31
  32. 32. AP Chemistry Rapid Learning Series - 15 Trigonal Bipyramidal Geometry AX5 5 electron regions Bonds: Inside “Triangle” = 120° Triangle 120 Between top/triangle/bottom = 90° Named: If each plane (defined by 3 points) is covered, 2 pyramids with triangular bases are sitting base-to-base. Examples: PCl5, AsF5 63/86 Octahedron Geometry AX6 6 electron regions Bonds: 90 90° Named: If each plane (defined by 3 points) is covered, it’s an 8 (“Octa”) sided object (“hedron”). Examples: SF6, PF6 64/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 32
  33. 33. AP Chemistry Rapid Learning Series - 15 Determining Electron Geometry Electron geometry is determined by electron regions. Electron regions Name of geometry Angle between regions 2 Linear 180° 3 Trigonal Planar 120° 4 Tetrahedral 109.5° 5 Trigonal Bipyramidal 90° and 120° 6 Octahedron Picture 90° 65/86 Determining Molecular Geometry Molecular geometry is determined by # of atoms bonded to the central atom. Formula Name of geometry Angle between bonded atoms AX2 Linear 180° AX3 Trigonal Planar 120° AX4 Tetrahedral 109.5 109 5° AX5 Trigonal Bipyramidal 90° and 120° AX6 Octahedron Picture 90° 66/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 33
  34. 34. AP Chemistry Rapid Learning Series - 15 Geometry with Lone Pairs Use the formula to determine the electron geometry. Each atom bonded to the central atom (ligand) counts as 1. Each lone pa cou ts as 1. ac o e pair counts e.g. AX2E2 = 2 bonds & 2 lone pairs = 4 electron regions To determine molecular geometry, first determine the electron geometry and then remove atoms to form lone pairs and re-name the geometry. 67/86 Bent Geometry with 1 Lone Pair AX2E Remove 1 atom Start with 3 electron regions Trigonal Planar Bent 120º Named: It looks like a “bent” line. Examples: SO2, O3 68/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 34
  35. 35. AP Chemistry Rapid Learning Series - 15 Bent Geometry with 2 Lone Pairs AX2E2 Remove 2 atoms Start with 4 electron regions Tetrahedron Bent 109.5º Named: It looks like a “bent” line. Examples: H2O, SF2 69/86 Trigonal Pyramidal Geometry AX3E Remove 1 atom Start with 4 electron regions Tetrahedron Trigonal Pyramidal Named: It looks like a pyramid with a triangular base. Examples: NH3, SOCl2 70/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 35
  36. 36. AP Chemistry Rapid Learning Series - 15 See-Saw Geometry AX4E Remove 1 atom Start with 5 electron regions Trigonal Bipyramidal See-Saw Named: It looks like a see-saw Examples: SF4, O2XeF2 71/86 T-Shaped Geometry AX3E2 Remove 2 atoms Start with 5 electron regions Trigonal Bipyramidal T-Shaped Named: It looks like a “T” Examples: ClF3, BrF3 72/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 36
  37. 37. AP Chemistry Rapid Learning Series - 15 Linear with 3 Lone Pairs AX2E3 Remove 3 atoms Start with 5 electron regions Trigonal Bipyramidal. Linear Named: It looks like a “line” Examples: XeF2, I3 73/86 Square Pyramidal Geometry AX5E Remove 1 atom Start with 6 electron regions Octahedron Square Pyramidal Named: It looks like a pyramid with a square base. Examples: BrF5, IF5 74/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 37
  38. 38. AP Chemistry Rapid Learning Series - 15 Square Planar Geometry AX4E2 Remove 2 atoms Start with 6 electron regions Octahedron Square Planar Named: It looks a flat (“planar”) square. Examples: ICl4, XeCl4 75/86 Effect of Lone Pairs on Molecular Geometry Both molecules have 4 electron regions: H H C H H H N H H These electrons are not being “controlled” by another nucleus. Lone pairs take up more space than a bonding pair—they distort the bond angles slightly. Instead of being 109.5° angles (tetrahedron), the angles are 107.3° 76/86 107.3° H N H H © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 107.3° 38
  39. 39. AP Chemistry Rapid Learning Series - 15 Determining Geometry with Lone Pairs Formula Name of geometry AX2E Bent AX2E2 Bent AX3E Trigonal Pyramidal AX4E See-Saw AX3E2 T-Shaped AX2E3 Linear AX5E Square Pyramidal AX4E2 Picture Square Planar 77/86 Geometry Example Example: Give the electron and molecular geometry for H2O. Lone pair 1 2 Bond H O H Bond 4 3 Lone Pair First, draw the Lewis Structure. For electron geometry, determine # of electron regions 4 electron regions = tetrahedral electron geometry g g y For molecular geometry, count atoms bonded to and lone pairs around the central atom. 2 bonded atoms and 2 lone pairs = bent molecular geometry 78/86 Note - this problem is commonly answered incorrectly as it “looks” linear as it’s written above. But the lone pairs make it bent! © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 39
  40. 40. AP Chemistry Rapid Learning Series - 15 Lewis Structures & The AP Exam 79/86 Lewis Structures in the Exam Common Lewis Structure problems: Draw Lewis Structures Identify VSEPR geometries Draw isomers or resonance structures Use Lewis Structures and bonding theory to explain behavior. 80/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 40
  41. 41. AP Chemistry Rapid Learning Series - 15 Multiple Choice Questions Problems often combine bonding theory with Lewis Structures. Example: E l Which Whi h VSEPR geometry h sp3d hybridization? t has h b idi ti ? A. Bent sp2 or sp3 B. Trigonal bipyramidal sp3d C. C Tetrahedron sp3 D. Octahedron sp3d2 Answer: B 81/86 Free Response Questions The Free Response questions often ask for you to explain certain behavior. Example: Explain the following in terms of atomic or molecular structure: A. The second ionization energy of Mg is much lower than the second ionization energy of Na B. The atomic radius of N is greater than O C. The boiling point of H2O is higher than H2S D. The bond angle of NH3 is smaller than NH4+ A Lewis Structure/Bonding theory question combined with other atomic structure, intermolecular forces sub-questions. 82/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 41
  42. 42. AP Chemistry Rapid Learning Series - 15 Answering Free Response Questions Drawing structures along with labeling and brief explanation is an excellent why to answer a question! D. D The bond angle of NH3 is smaller than NH4+ Lone pairs take up more space than bonding pairs. H H N H H H N H H This distorts the other bond angles in towards each other. 83/86 Learning Summary Valence Shell Electron Pair Repulsion (VSEPR) theory is used to th i dt determine geometry of molecules. Elements bond to obtain a full valence shell— for most elements, that means 8 (the octet rule). Lewis Structures are used to show the valence electrons and their arrangement in compounds. Electron geometry is determined by electron regions, while molecular geometry is determined by atoms bonded to the central atom. Covalent compounds share electrons, while ionic compounds transfer electrons from one atom to another. 84/86 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 42
  43. 43. AP Chemistry Rapid Learning Series - 15 Congratulations You have successfully completed the core tutorial Lewis Structures Rapid Learning Center 85/86 Rapid Learning Center Chemistry :: Biology :: Physics :: Math What’s N t Wh t’ Next … Step 1: Concepts – Core Tutorial (Just Completed) Step 2: Practice – Interactive Problem Drill Step 3: Recap – Super Review Cheat Sheet Go for it! 86/86 http://www.RapidLearningCenter.com © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 43

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