CHE 205
General Chemistry
Faculty of Engineering
Dr. Sara El Moussawi
Fall 2022/2023
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Chapter II: Bonding : General Concepts
Chapter 2: Bonding: General Concepts 2
Chapter 2
Bonding: General Concepts
Chapter outline
2.1 Types of Chemical Bonds
2.2 Electronegativity
2.3 Bond Polarity and Dipole Moments
2.4 Ions: Electron Configurations and Sizes
2.5 Partial Ionic Character of Covalent Bonds
2.6 The Covalent Chemical Bond: A Model
2.7 The Localized Electron Bonding Model
2.8 Lewis Structures
2.9 Exceptions to the Octet Rule
2.10 Resonance
2.11 Formal Charge
2.12 Molecular Structure: The VSEPR Model
Chapter 2: Bonding: General Concepts 3
Molecular Structure: The VSEPR Model
Valence Shell Electron-Pair Repulsion (VSEPR) Model, is useful in predicting the geometries of
molecules formed by covalent bonds
The structure around a given atom is determined principally by minimizing electron- pair repulsions.
The idea here is that the bonding and nonbonding pairs around a given atom will be positioned as far
apart as possible.
Repulsion between Electron Groups
determine molecular geometry
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Chapter 2: Bonding: General Concepts
Molecular Structure: The VSEPR Model
Consider the molecule BeCl2:
Lewis structure:
BeCl2 has two single bonds about the central atom = two electron groups
According to VSEPR theory → minimum repulsion between these two electron groups →
maximum separation possible → separation by 180° bond angle or a linear geometry.
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Chapter 2: Bonding: General Concepts
Molecular Structure: The VSEPR Model
Consider BF3:
The boron atom is surrounded by
Lewis structure: three pairs of electrons
• The electron pairs are farthest apart at angles of 120 degrees:
This is a planar (flat) and triangular
molecule, it is
Trigonal planar structure.
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Chapter 2: Bonding: General Concepts
Molecular Structure: The VSEPR Model
Consider methane molecule:
Lewis structure: There are four pairs of electrons
around the central carbon atom
• Try a square planar arrangement (atoms are in the plane of the paper, and the angles between
the pairs are all 90 degrees):
But, there is another arrangement with angles
greater than 90 degrees that would put the
electron pairs even farther away from each
other.
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Chapter 2: Bonding: General Concepts
Molecular Structure: The VSEPR Model
- The Tetrahedral structure has angles of 109.5 degrees:
- This is the maximum possible separation of four pairs around a given atom.
Whenever four pairs of electrons are present around an atom, they should always
be arranged tetrahedrally.
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Chapter 2: Bonding: General Concepts
Molecular Structure: The VSEPR Model
Consider PCl5 : • There are five pairs of electrons around the
central phosphorous atom
Lewis structure:
• Five electron groups around a central atom
assume a Trigonal bipyramidal geometry
• The angles in the trigonal bipyramidal structure are not all the same.
• The angles between the equatorial positions (the three bonds in the trigonal plane) are 120°
• the angle between the axial positions (the two bonds on either side of the trigonal plane) and the
trigonal plane is 90°.
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Chapter 2: Bonding: General Concepts
Molecular Structure: The VSEPR Model
Consider SF6:
• There are six pairs of electrons around the
central sulfur atom
Lewis structure:
• Six electron groups around a central atom
assume an Octahedral geometry
The angles in this
geometry are all 90°.
We can see that the structure of this molecule is highly symmetrical. All six bonds are
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equivalent.
Molecular Structure: The VSEPR Model
Arrangements of Electron
Pairs Around an Atom
Yielding Minimum Repulsion.
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Molecular Structure: The VSEPR Model
Exercise:
Solution:
Linear. HCN has two electron groups (the single bond and the triple bond) resulting in a linear geometry.
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Molecular Structure: The VSEPR Model
Exercise:
Determine the molecular geometry of NO-3
Solution:
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Molecular Structure: The VSEPR Model
Steps to Apply the VSEPR Model
1. Draw the Lewis structure for the molecule.
2. Count the electron pairs and arrange them in the way that minimizes repulsion
3.Determine the positions of the atoms from the way the electron pairs are shared.
4. Determine the name of the molecular structure from the positions of the atoms.
NB: a single bond, a double bond, or a triple bond are considered as a single electron group
What about the lone pairs of a central atom?
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Molecular Structure: The VSEPR Model
The Effect of Lone Pairs
• Each of the examples we examined until now has only bonding electron groups around the central atom.
What happens in molecules that have lone pairs around the central atom as well?
• These lone pairs also repel other electron groups → additional electron region
Consider the NH3:
1 Lone pair
1. Lewis structure:
2. The NH3 molecule has Four pairs of electrons:
• three bonding pairs
• one nonbonding pair (lone pair)
→Four electron pairs is Tetrahedral
3 Bonding pairs
3. The three H atoms share electron pairs.
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Molecular Structure: The VSEPR Model
Electron geometry vs. Molecular geometry
3 atoms
4 electron
around
groups
the center
The electron geometry The molecular geometry
Tetrahedral Trigonal pyramidal
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Molecular Structure: The VSEPR Model
Electron geometry vs. Molecular geometry
4. The name of the molecular structure is always based on the positions of the
atoms.
• The placement of the electron pairs determines the structure, but the name is based on
the positions of the atoms.
• NH3 has a tetrahedral arrangement of electron pairs but not a tetrahedral arrangement of
atoms.
• The molecular structure of ammonia is a trigonal pyramid (one side is different from
the other three) rather than a tetrahedron.
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Molecular Structure: The VSEPR Model
• “General Rule”
• Electron groups repulsion
The molecular structure of the water molecule:
The Lewis structure for water is:
4 electron groups → 2 bonding pairs and 2 nonbonding pairs.
To minimize repulsions, these are electron groups are arranged in a tetrahedral array
But what is the molecular geometry? 18
Molecular Structure: The VSEPR Model
• The molecular structure of the water molecule:
The electron geometry The molecular geometry
Tetrahedral V-shaped or bent
• The H2O molecule is V-shaped, or bent, because of the presence of the lone pairs.
• If no lone pairs were present, the molecule would be linear, the polar bonds would cancel, and the
molecule would have no dipole moment.
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Molecular Structure: The VSEPR Model
• The H-X-H bond angle (where X is the central atom) in CH4, NH3, and H2O should be the tetrahedral
angle of 109.5 degrees
.
• Experimental studies, however, show that the actual bond angles are:
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Molecular Structure: The VSEPR Model
• The table below summarizes the structures possible for molecules in which there are four
electron groups around the central atom with various numbers of atoms bonded to it.
Structures of Molecules with Four Electron Pairs Around the Central Atom.
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Molecular Structure: The VSEPR Model
Structures of
Molecules with
Five Electron Groups with lone pairs Five Electron Pairs
Around the
• Axial position (90⁰) and Equatorial position (120⁰) Central Atom.
• For minimal repulsion, lone pairs occupy the Equatorial position
• Table summarizes the structures possible for molecules in which
there are five electron groups around the central atom with
various numbers of atoms bonded to it.
• - The electron geometry that produces minimum repulsion is a
trigonal bipyramid.
• - The name of the molecular geometry depends on the position
of atoms around the central atom.
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Molecular Structure: The VSEPR Model
Six Electron Groups with Lone Pairs
• Consider the BrF5:
Lewis structure:
The electron geometry, due to the six electron
groups, is octahedral.
• Since all six positions in the octahedral geometry are equivalent, the lone pair can be situated in any
one of these positions. The resulting molecular geometry is square pyramidal.
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Molecular Structure: The VSEPR Model
Six Electron Groups with Lone Pairs
Consider the XeF4:
• When two of the six electron groups around the central atom are lone pairs, as in XeF4, the lone
pairs occupy positions across from one another (to minimize lone pair– lone pair repulsions),
and the resulting molecular geometry is square planar.
• Although each Xe-F bond is polar (fluorine has a greater electronegativity than xenon), the
square planar arrangement of these bonds causes the polarities to cancel.
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Molecular Structure: The VSEPR Model
Exercise:
When phosphorus reacts with excess chlorine gas, the compound phosphorus pentachloride
(PCl5) is formed. In the gaseous and liquid states, this substance consists of PCl5 molecules , but
in the solid state it consists of a 1:1 mixture of PCl4 +, and PCl6 - ions. Predict the geometric structures
of PCl5 , PCl4 +, and PCl6 -.
Solution:
The Lewis structure for PCl5 is shown below.
Five pairs of electrons around the phosphorus atom require a trigonal bipyramidal
arrangement (see Table 3).
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Molecular Structure: The VSEPR Model
• The Lewis structure for the PCl4 + ion [5 + 4(7) - 1 = 32 valence electrons] is shown below.
• There are four pairs of electrons surrounding the phosphorus atom in the PCl4 + ion, which
requires a tetrahedral arrangement of the pairs.
• Since each pair is shared with a chlorine atom, a tetrahedral PCl4 + cation results.
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Molecular Structure: The VSEPR Model
The Lewis structure for PCl6 - [5 + 6(7) + 1 = 48 valence electrons] is shown below.
Since phosphorus is surrounded by six pairs of electrons, an octahedral arrangement
is required to minimize repulsions.
Since each electron pair is shared with a chlorine atom, an octahedral PCl6 anion is predicted.
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Molecular Structure: The VSEPR Model
Molecules Containing No Single Central Atom
Consider methanol (CH3OH):
Lewis structure:
The molecular structure can be predicted from the arrangement of electron pairs around the carbon and
oxygen atoms.
• There are four pairs of electrons around the carbon, which requires a tetrahedral arrangement.
• The oxygen also has four electron pairs, which requires a tetrahedral arrangement (bent molecular
geometry).
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Molecular Structure: The VSEPR Model
Review
The rules for using the VSEPR model to predict molecular structure are as follows:
• Determine the Lewis structure(s) for the molecule.
• For molecules with resonance structures, use any of the structures to predict the molecular structure.
• Sum the electron pairs around the central atom.
• In counting pairs, count each multiple bond as a single effective pair.
• The arrangement of the pairs is determined by minimizing electron-pair repulsions. These
arrangements are shown in Table 1.
• Lone pairs require more space than bonding pairs do. Choose an arrangement that gives the lone
pairs as much room as possible.
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Molecular Structure: The VSEPR Model
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Molecular Structure: The VSEPR Model
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Molecular Structure: The VSEPR Model
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Molecular Structure: The VSEPR Model
Exercise:
A molecule with the formula AB3 has a trigonal pyramidal geometry. How many electron groups are on the
central atom (A)?
Solution:
The molecule has 4 electron groups around the center, example: NH3
3 atoms
4 electron
around
groups
the center
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Molecular Structure: The VSEPR Model
Exercise:
For each molecular geometry, determine the number of total electron groups, the number of bonding groups,
and the number of lone pairs on the central atom.
Solution:
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Molecular Structure: The VSEPR Model
Exercise:
Determine the electron geometry, molecular geometry for each molecule:
a. PF3 b. CH4 c. SBr2
Solution:
molecular geometry:
a. PF3 trigonal pyramidal
Lewis structure:
According to Lewis structure the are 4 electron groups: 3 bonding pairs and one lone pair → The electron
geometry is tetrahedral and the molecular geometry: trigonal pyramidal
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Molecular Structure: The VSEPR Model
b. CH4 molecular geometry:
tetrahedral
Lewis structure:
According to Lewis structure the are 4 electron groups: 4 bonding pairs → Both the electron geometry and the
molecular geometry are tetrahedral
c. SBr2
molecular geometry:
Lewis structure: Bent
According to Lewis structure the are 4 electron groups: 2 bonding pairs and two lone pairs → The electron geometry
is tetrahedral and the molecular geometry: Bent 36
