 # Section 3.3 – Molecular Shapes and Dipoles It is time for these molecules to get in shape! Nelson: pages 91-104.

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Section 3.3 – Molecular Shapes and Dipoles It is time for these molecules to get in shape!
Nelson: pages

Molecular Shape Lewis structures tell us nothing about how atoms in a molecule are arranged in 3-dimensional space Could you have predicted the arrangement of atoms on the right from just seeing it’s Lewis structure?

VSEPR “vesper” THEORY Shapes of molecules:
Molecules take on particular shapes depending on the amount of lone pairs and bonding electrons the atom in the middle of the molecule has Stereochemistry – study of the 3-D shape of molecules and how it affects their physical and chemical properties VSEPR = “valence- shell-electron-pair-repulsion” theory is based on: Electron pairs try to stay as far as possible from other e due to repulsion of negative charges The number, type and direction of bonds to the central atom (CA) determine the shape of the molecule

VSEPR Continued… VSEPR Theory is a powerful tool that helps us to guess the shape of a molecule. Such shapes are important as they determine the structure and function of the compound. For example, the arrangement of carbon atoms in a diamond help establish its unique hardness and usefulness.

According to VSEPR theory:
Read and summarize the points on page 91 in the space below:

Using VSEPR to predict molecular shapes
Bond angles – the angle formed by 2 bonds intersecting at an atom A few rules to predicting the shape of atoms get electron pairs as far away as possible (like charges repel) multiple bonds are treated as 1 bond lone pairs take slightly more room than bonding pairs

VSEPR Continued… There are 6 different arrangements that you will need to memorize!!! Let’s look at specific examples of molecules to help us come up with general formulas for each of the arrangements Let A = Central atom Let X = bonding electrons (bond pair) Let E = one pair Remember: the key concept is that all pairs of valence e repel each other and try to get as far from each other as possible Steps to determining shape: draw a L.D.D and consider the arrangement of valence e’s: Determine bond pairs and lone pairs around central atom (CA)

Consider beryllium dihydride Total pairs = 2
Bond pairs around Be (CA) = 2 Lone pairs around CA = 0 Bond pairs repel each other and try to get as far away as possible = opposite sides of Be Gives a linear orientation with the two bonds at an angle of 180 º Summary: General Formula Bond pairs Lone pairs Total pairs Electron pair arrangement Stereochemical formula AX2 linear

VSEPR: Linear Arrangement

boron trifluoride Lewis dot diagram: Total pairs = 3 Lone pairs = 0
Bonding pairs = 3 Repulsion of bonding pairs causes 120º bond angles Summary: General Formula Bond pairs Lone pairs Total pairs Electron pair arrangement Stereochemical formula AX3 Trigonal Planar

VSEPR: Trigonal Planar Arrangement

Drawing in 3-D In the plane of the slide Going back and away from you
tetrahedral Coming out toward you

carbon tetrachloride Lewis dot diagram: Total pairs = 3 Lone pairs = 0
Bonding pairs = 3 Repulsion of bonding pairs causes º bond angles Summary: General Formula Bond pairs Lone pairs Total pairs Electron pair arrangement Stereochemical formula AX4 tetrahedral

VSEPR: Tetrahedral Arrangement

nitrogen trichloride Lewis dot diagram: Total pairs = 4 Lone pairs = 1
Bonding pairs = 3 Repulsion of electron pairs causes a tetrahedral shape If we ignore the lone pair, the shape becomes like a 3 sided (triangular) pyramid = trigonal pyramidal We would predict that the bond angles would be 109.5º like the tetrahedral arrangement. However, lone pairs have a greater repulsion that bond pairs and therefore pushes that bond pair angles to be 107.3º General Formula Bond pairs Lone pairs Total pairs Electron pair arrangement Stereochemical formula AX3E tetrahedral trigonal pyramidal

Water Lewis dot diagram: Total pairs = 4 Lone pairs = 2
Bonding pairs = 2 Repulsion of bonding pairs causes a slightly altered (due to lone pairs) tetrahedral bonding pattern with the bond pairs having a bond angle of 104.5º Summary: General Formula Bond pairs Lone pairs Total pairs Electron pair arrangement Stereochemical formula AX2E2 tetrahedral Angular (V-shaped)

bonding-pair vs. bonding
pair repulsion lone-pair vs. lone pair repulsion lone-pair vs. bonding >

Hydrogen fluoride Lewis dot diagram: Total pairs = 4 Lone pairs = 3
Bonding pairs = 1 Repulsion of bonding pairs causes a tetrahedral bonding pattern. Since there is only two atoms held together by one covalent bond, the shape is linear (like all diatomic molecules) Summary: General Formula Bond pairs Lone pairs Total pairs Electron pair arrangement Stereochemical formula AXE3 tetrahedral Linear

Table 7: Using VSEPR to predict molecular shape – Nelson Page 95
General Formula Bond Pairs Lone Pairs Total Pairs Geometry Stereochemical formula Examples:

General Formula Bond Pairs Lone Pairs Total Pairs Geometry Stereochemical formula Examples:

Read the Learning Tips on pages 92, 94, and 96 Try Practice Problems on Page 96 # 2-4 = _____________________? = _____________________? = _____________________?

Practice Problems Page 96- Solutions

Everyone wants to be the center of the universe
If you have more then one CA you follow three simple steps start with a structural or lewis diagram identity all the CA and treat each one individually draw the molecule going from CA to CA Example – C2H6

More than one CA Continued…
Example: What are the bond pairs and lone pairs around C in the following molecule? Around N? Draw a three dimensional structure for the compound. C: 4 bond pairs tetrahedral N: 3 bond pairs, 1 lone pair trigonal pyramidal

VSEPR For the previous compound, two realistic 3-D structures would be:

Multiple bonds and VSEPR
Can we predict stereochemistry for molecules with multiple bonds? Consider ethylene (used in welding torches): C2H4 (g) Crystallography indicates that the orientation around the C atoms is trigonal planer Just as before with multiple CA’s: Step 1: draw L.D.D Step 2: count bond and lone pairs around CA (Recall: double/ triple bonds count as one) Step 3: Determine general formula for each CA: AX3 Step 4: draw a structural diagram if necessary

In acetic acid, CH3COOH, there are three central atoms, and one double bond.
3-D molecule applet

VSEPR Example: Draw a three dimensional structure for the following compound. First, determine the bond and lone pairs around each CA Trigonal planar tetrahedral

More common way to draw structure.
VSEPR Two possible 3-D structures: More common way to draw structure.

Learning Activities: Read Pages 91- 97 from Nelson
Finish Practice Problems #2-4 on page 96 Try Practice Problems 6-7 on page 98 Complete Section 3.3 Questions 1-3 on page 104

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