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CHAPTER 7
7.3 Molecular Geometry and
Lewis Dot Structures
Bonding
2 7.3 Molecular Geometry and Lewis Dot Structures
Each water molecule contains one oxygen atom and two hydrogen atoms.
One central oxygen atom
One hydrogen atom on either side
3 7.3 Molecular Geometry and Lewis Dot Structures
Each water molecule contains one oxygen atom and two hydrogen atoms.
One central oxygen atom
One hydrogen atom on either side
Why can’t a water molecule be like this?
4 7.3 Molecular Geometry and Lewis Dot Structures
Why can’t a water molecule be like this?
The oxygen forms one bond
One hydrogen forms two bonds
One hydrogen forms one bond
The Lewis structures indicate that it is not possible
5 7.3 Molecular Geometry and Lewis Dot Structures
Lewis structures for individual atoms are like puzzle pieces.Put them together to form molecules.
6 7.3 Molecular Geometry and Lewis Dot Structures
Use Lewis structures to predict:
1) the chemical formula
The chemical formula for water is H2O
(2 hydrogen atoms for every 1 oxygen atom)
7 7.3 Molecular Geometry and Lewis Dot Structures
Use Lewis structures to predict:
1) the chemical formula
2) the bonding pattern
Oxygen must be the central atom
8 7.3 Molecular Geometry and Lewis Dot Structures
Use Lewis structures to predict:
1) the chemical formula
2) the bonding pattern
3) the shape of the molecule
H2O is flat and bent
9 7.3 Molecular Geometry and Lewis Dot Structures
Use Lewis structures to predict:
1) the chemical formula
2) the bonding pattern
3) the shape of the molecule
H2O is flat and bent
To be discussed later in this section
10 7.3 Molecular Geometry and Lewis Dot Structures
Consider the chemical formula C2H6O
Does this look right?
11 7.3 Molecular Geometry and Lewis Dot Structures
Ethanol
Consider the chemical formula C2H6O
12 7.3 Molecular Geometry and Lewis Dot Structures
Consider the chemical formula C2H6O
Could this be right
too?
13 7.3 Molecular Geometry and Lewis Dot Structures
Consider the chemical formula C2H6O
Dimethyl ether
14 7.3 Molecular Geometry and Lewis Dot Structures
isomer: a specific structure of a molecule, only used when a chemical formula could represent more than one molecule.
Dimethyl etherEthanol
Two isomers of C2H6O
15 7.3 Molecular Geometry and Lewis Dot Structures
Give three isomers for the formula C3H8O. Show the Lewis dot diagram and the structural formula for each molecule.
16 7.3 Molecular Geometry and Lewis Dot Structures
Give three isomers for the formula C3H8O. Show the Lewis dot diagram and the structural formula for each molecule.
Asked: The Lewis dot diagrams and structural formulas for the three molecules represented by the formula C3H8O
Given: Carbon has four unpaired electrons, hydrogen has one, and oxygen has two. Three carbons, eight hydrogens and one oxygen form each molecule.
Relationships: The atoms will bond together such that all unpaired electrons will be paired up with electrons from other atoms.
17 7.3 Molecular Geometry and Lewis Dot Structures
Give three isomers for the formula C3H8O. Show the Lewis dot diagram and the structural formula for each molecule.
Asked: The Lewis dot diagrams and structural formulas for the three molecules represented by the formula C3H8O
Given: Carbon has four unpaired electrons, hydrogen has one, and oxygen has two. Three carbons, eight hydrogens and one oxygen form each molecule.
Relationships: The atoms will bond together such that all unpaired electrons will be paired up with electrons from other atoms.
Solve:
18 7.3 Molecular Geometry and Lewis Dot Structures
Ethene Ethyne
Multiple bonds
Carbon, nitrogen and oxygen commonly form double and triple bonds.
Double bond (2 pairs of electrons)
Sharing a pair of electrons is called a single bond.
Triple bond (3 pairs of electrons)
19 7.3 Molecular Geometry and Lewis Dot Structures
Bond atoms together, forming single bonds.
Acetonitrile (C2H3N)
20 7.3 Molecular Geometry and Lewis Dot Structures
Bond atoms together, forming single bonds.
Form double bonds by bringing together single electrons from atoms that are already bonded.
Acetonitrile (C2H3N)
21 7.3 Molecular Geometry and Lewis Dot Structures
Bond atoms together, forming single bonds.
Form double bonds by bringing together single electrons from atoms that are already bonded.
Form triple bonds with any remaining single electrons.
Acetonitrile (C2H3N)
22 7.3 Molecular Geometry and Lewis Dot Structures
Bond atoms together, forming single bonds.
Form double bonds by bringing together single electrons from atoms that are already bonded.
Form triple bonds with any remaining single electrons.
Write final Lewis dot structure and structural formula.
Acetonitrile (C2H3N)
23 7.3 Molecular Geometry and Lewis Dot Structures
Bond atoms together, forming single bonds.
Formaldehyde (CH2O)
24 7.3 Molecular Geometry and Lewis Dot Structures
Bond atoms together, forming single bonds.
Form double bonds by bringing together single electrons from atoms that are already bonded.
Formaldehyde (CH2O)
25 7.3 Molecular Geometry and Lewis Dot Structures
Bond atoms together, forming single bonds.
Form double bonds by bringing together single electrons from atoms that are already bonded.
Form triple bonds with any remaining single electrons.
Write final Lewis dot structure and structural formula.
Formaldehyde (CH2O)
26 7.3 Molecular Geometry and Lewis Dot Structures
Use Lewis structures to predict:
1) the chemical formula
2) the bonding pattern
3) the shape of the molecule
H2O is flat and bent
To be discussed later in this section
27 7.3 Molecular Geometry and Lewis Dot Structures
The lone pairs of electrons are not involved
in bonding, but affect the shape of the molecule.
Valence
Shell
Electron
Pair
Repulsion
H2O is flat and bent
28 7.3 Molecular Geometry and Lewis Dot Structures
Valence
Shell
Electron
Pair
Repulsion
H2O is flat and bent
VSEPR: a theory that states that the shapes of molecules are dictated, in part, by the repulsion of the shared electrons and the unshared pairs of electrons.
29 7.3 Molecular Geometry and Lewis Dot Structures
Similar charges repel each other.
Identify regions of electron density to predict the molecular geometry.
Carbon dioxide
2 REGIONS
30 7.3 Molecular Geometry and Lewis Dot Structures
Similar charges repel each other.
Identify regions of electron density to predict the molecular geometry.
Carbon dioxide
2 REGIONS
Formaldehyde
3 REGIONS
31 7.3 Molecular Geometry and Lewis Dot Structures
Similar charges repel each other.
Identify regions of electron density to predict the molecular geometry.
Carbon dioxide Formaldehyde Methane
2 REGIONS 3 REGIONS 4 REGIONS
32 7.3 Molecular Geometry and Lewis Dot Structures
Two regions
Two charged balloons repel in opposite directions
Two areas of electron density repel to form linear shapes
33 7.3 Molecular Geometry and Lewis Dot Structures
Two regions
Two charged balloons repel in opposite directions
Two areas of electron density repel to form linear shapes
These two regions of electron density repel each other,
forming a 180o angle
34 7.3 Molecular Geometry and Lewis Dot Structures
Two regions
Two charged balloons repel in opposite directions
Two areas of electron density repel to form linear shapes
These two regions of electron density repel each other,
forming a 180o angle
35 7.3 Molecular Geometry and Lewis Dot Structures
Two areas of electron density repel to form linear shapes
The two 180o angles formed around each carbon make the entire molecule straight.
Two regions
36 7.3 Molecular Geometry and Lewis Dot Structures
Two regions
There are two isomers for the formula C3H4. Show the Lewis dot diagram for each molecule, and indicate which atoms are at the center of a linear part of the molecules.
37 7.3 Molecular Geometry and Lewis Dot Structures
Two regions
There are two isomers for the formula C3H4. Show the Lewis dot diagram for each molecule, and indicate which atoms are at the center of a linear part of the molecules.
Asked: The linear parts of each isomer of C3H4
Given: There are two different isomers. Part of each molecule will be linear. The molecules are made from three carbons and four hydrogens.
Relationships: Each atom that has two regions of electron density around it will form a linear part of the molecule.
38 7.3 Molecular Geometry and Lewis Dot Structures
Two regions
There are two isomers for the formula C3H4. Show the Lewis dot diagram for each molecule, and indicate which atoms are at the center of a linear part of the molecules.
Asked: The linear parts of each isomer of C3H4
Given: There are two different isomers. Part of each molecule will be linear. The molecules are made from three carbons and four hydrogens.
Relationships: Each atom that has two regions of electron density around it will form a linear part of the molecule.
Solve:
39 7.3 Molecular Geometry and Lewis Dot Structures
Three regions
Three charged balloons repel into the corners
of a triangle
Three areas of electron density repel to form trigonal planar shapes
40 7.3 Molecular Geometry and Lewis Dot Structures
Three regions
Three charged balloons repel into the corners
of a triangle
Three areas of electron density repel to form trigonal planar shapes
These three regions of electron density repel, forming 120o angles between the three atoms bonded to each carbon atom
41 7.3 Molecular Geometry and Lewis Dot Structures
Three regions
Three charged balloons repel into the corners
of a triangle
Three areas of electron density repel to form trigonal planar shapes
These three regions of electron density repel, forming 120o angles between the three atoms bonded to each carbon atom
42 7.3 Molecular Geometry and Lewis Dot Structures
Three areas of electron density repel to form trigonal planar shapes
Three regions
These three regions of electron density repel, forming 120o angles between the three atoms bonded to each carbon atom
43 7.3 Molecular Geometry and Lewis Dot Structures
Acetic acid when mixed with water is commonly known as vinegar and has the formula C2H4O2. The correct isomer has both oxygens bonded to the same carbon. Draw the Lewis dot structure for this isomer and indicate where the molecule will be trigonal planar.
Three regions
44 7.3 Molecular Geometry and Lewis Dot Structures
Acetic acid when mixed with water is commonly known as vinegar and has the formula C2H4O2. The correct isomer has both oxygens bonded to the same carbon. Draw the Lewis dot structure for this isomer and indicate where the molecule will be trigonal planar.
Three regions
Asked: The trigonal planar parts of acetic acid
Given: The formula for acetic acid is C3H4O2 and both oxygens are bonded to the same carbon.
Relationships: Each atom that has three regions of electron density around it will form a trigonal planar part of the molecule.
45 7.3 Molecular Geometry and Lewis Dot Structures
Acetic acid when mixed with water is commonly known as vinegar and has the formula C2H4O2. The correct isomer has both oxygens bonded to the same carbon. Draw the Lewis dot structure for this isomer and indicate where the molecule will be trigonal planar.
Three regions
Asked: The trigonal planar parts of acetic acid
Given: The formula for acetic acid is C3H4O2 and both oxygens are bonded to the same carbon.
Relationships: Each atom that has three regions of electron density around it will form a trigonal planar part of the molecule.
Solve:
46 7.3 Molecular Geometry and Lewis Dot Structures
Four regions
Four charged balloon repel into the corners of a
tetrahedron, rather than lying flat in a plane
The four regions of electron density around
the carbon repel, forming angles of 109.5o.
47 7.3 Molecular Geometry and Lewis Dot Structures
Four regions
The four regions of electron density around
the carbon repel, forming angles of 109.5o.
48 7.3 Molecular Geometry and Lewis Dot Structures
Four regions
Ammonia (NH3) forms a trigonal pyramidal
shape
Lone pair of electrons
The lone pair of electrons repels the shared electrons in the H–N bonds, pushing
the hydrogens away from the lone pair
Lone pairs of electrons repel just like shared pairs
49 7.3 Molecular Geometry and Lewis Dot Structures
Four regions
Different geometries formed by atoms with four regions of electron density
Trigonal pyramidal
Tetrahedral
Bent
50 7.3 Molecular Geometry and Lewis Dot Structures
What shapes are formed within the isomer of C4H5NO, which has a triple bond connecting nitrogen?
51 7.3 Molecular Geometry and Lewis Dot Structures
Four regions
What shapes are formed within the isomer of C4H5NO, which has a triple bond connecting nitrogen?
Solve:
52 7.3 Molecular Geometry and Lewis Dot Structures
Use Lewis structures to predict:
1) the chemical formula
2) the bonding pattern
3) the shape of the molecule
