Interatomic Forces and Valence Bond Theory
Lecture 5
Bonding Forces and Energies
1
Recap
In last lecture Secondary Bonding was discussed.
They are weaker in nature and are broadly classified as Van der Waal's
forces, hydrogen bonds and London dispersion forces.
These bonds are due to atomic or molecular dipoles, both permanent and
temporary.
In non -po
lar molecules temporary dipoles exists which gives rise to
week forces.
2
Interatomic Forces
The forces between the atoms due to electrostatic interaction between the charges
of the atoms are called interatomic forces.
These forces are electrical in nature and these are active if the distance between the
two atoms is of the order of atomic size i.e. 10
–10
metre.
As they approach each other, the following
interactions are observed.
1. Attractive force A between the nucleus of one
atom and electron of the other. This attractive force
tends to decrease the potential energy of the atomic
system.
2. Repulsive force R between the nucleus of one
atom and the nucleus of the other atom and electron of
one atom with the electron of the other atom. These
repulsive forces always tend to increase the energy of
the atomic system.
Figure 1
3
Interatomic Forces
If the net effect of the forces of attraction and repulsion leads to decrease in
the energy of the system, the two atoms come closer to each other and form a
covalent bond by sharing of electrons. On the other hand, if the repulsive forces
are more and there is increase in the energy of the system, the atoms will repel
each other and do not form a bond.
There is a universal tendency of all systems to acquire a state of minimum
potential energy. This stage of minimum potential energy corresponds to
maximum stability.
4
Valence Bond Theory
Valence Bond Theory
were developed to use the methods of quantum mechanics
to explain chemical bonding.
It focuses on how the atomic orbitals of the dissociated atoms combine to give
individual chemical bonds when a molecule is formed.
The simplest case to consider is the hydrogen molecule, H
2
When we say that the two hydrogen nuclei share their electrons to form a covalent
bond, what we mean in VB theory terms is that the two spherical 1
s
orbitals (the grey
spheres in Figure 1 overlap and contain two electron
Figure 2
These two electrons are now attracted to the
positive charge of both of the hydrogen nuclei,
with the result that they serve as a sort of
‘chemical glue’ holding the two nuclei together.
5
Valence Bond Theory
How far apart are the two nuclei?
If they are too far apart, their respective 1
s
orbitals cannot overlap, and thus
no covalent bond can form - they are still just two separate hydrogen atoms.
As they move closer and closer together, orbital overlap begins to occur, and
a bond begins to form.
This lowers the
potential energy
of the system, as new,
attractive
positive-
negative electrostatic interactions become possible between the nucleus of one
atom and the electron of the second .
However, something else is happening at the same time: as the atoms get
closer, the
repulsive
positive-positive interaction between the two nuclei also
begins to increase.
6
Potential Energy Vs Interatomic Distance
At long distances, both attractive and
repulsive interactions are small.
As the distance between the atoms
decreases, the attractive electron–
proton interactions dominate, and the
energy of the system decreases.
At the observed bond distance, the
repulsive electron–electron and proton–
proton interactions just balance the
attractive interactions, preventing a
further decrease in the internuclear
distance.
At very short internuclear distances,
the repulsive interactions dominate,
making the system less stable than the
isolated atoms.
Figure 3
7
Potential Energy Vs Interatomic Distance
8
Potential Energy Vs Interatomic Distance
At first this repulsion is more than offset by the attraction between nuclei and
electrons, but at a certain point, as the nuclei get even closer, the repulsive forces
begin to overcome the attractive forces, and the potential energy of the system rises
quickly.
When the two nuclei are ‘too close’, we have an unstable, high-energy situation.
There is a defined optimal distance between the nuclei in which the potential energy
is at a minimum, meaning that the combined attractive and repulsive forces add up to
the greatest overall attractive force.
This optimal internuclear distance is the
bond length
; the distance is 74 pm for H
2
9
Potential Energy Vs Interatomic Distance
Likewise, the difference in potential energy between the lowest energy state (at the
optimal internuclear distance) and the state where the two atoms are completely separated
is called the
bond dissociation energy,
or, more simply
, bond strength
; the
H
2
bond
strength is 435 kJ/mol.
The amount of energy required to break a bond is called
bond dissociation energy
or
simply
bond energy
.
Bond energy is a measure of the strength of a chemical bond. The larger the bond energy, the
stronger the bond.
10
Summary
Potential Energy Vs Interatomic Distance graph was discussed
.
At long distances, both attractive and repulsive interactions are small.
As the distance between the atoms decreases, the attractive electron–proton
interactions dominate, and the energy of the system decreases.
At the observed bond distance, the repulsive electron–electron and proton–
proton interactions just balance the attractive interactions, preventing a further
decrease in the internuclear distance.
At very short internuclear distances, the repulsive interactions dominate, making
the system less stable than the isolated atoms.
11
Forces between atoms, such as electrostatic interactions and covalent bonding explained through Valence Bond Theory. Learn about secondary bonding and the formation of chemical bonds.
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Presentation Transcript
Lecture 5 Bonding Forces and Energies 1
Recap In last lecture Secondary Bonding was discussed. They are weaker in nature and are broadly classified as Van der Waal's forces, hydrogen bonds and London dispersion forces. These bonds are due to atomic or molecular dipoles, both permanent and temporary. In non -polar molecules temporary dipoles exists which gives rise to week forces. 2
Interatomic Forces The forces between the atoms due to electrostatic interaction between the charges of the atoms are called interatomic forces. These forces are electrical in nature and these are active if the distance between the two atoms is of the order of atomic size i.e. 10 10 metre. As they approach each other, the following interactions are observed. 1. Attractive force A between the nucleus of one atom and electron of the other. This attractive force tends to decrease the potential energy of the atomic system. 2. Repulsive force R between the nucleus of one atom and the nucleus of the other atom and electron of one atom with the electron of the other atom. These repulsive forces always tend to increase the energy of the atomic system. Figure 1 3
Interatomic Forces If the net effect of the forces of attraction and repulsion leads to decrease in the energy of the system, the two atoms come closer to each other and form a covalent bond by sharing of electrons. On the other hand, if the repulsive forces are more and there is increase in the energy of the system, the atoms will repel each other and do not form a bond. There is a universal tendency of all systems to acquire a state of minimum potential energy. This stage of minimum potential energy corresponds to maximum stability. 4
Valence Bond Theory Valence Bond Theory were developed to use the methods of quantum mechanics to explain chemical bonding. It focuses on how the atomic orbitals of the dissociated atoms combine to give individual chemical bonds when a molecule is formed. The simplest case to consider is the hydrogen molecule, H2 When we say that the two hydrogen nuclei share their electrons to form a covalent bond, what we mean in VB theory terms is that the two spherical 1s orbitals (the grey spheres in Figure 1 overlap and contain two electron These two electrons are now attracted to the positive charge of both of the hydrogen nuclei, with the result that they serve as a sort of chemicalglue holding the two nuclei together. Figure 2 5
Valence Bond Theory How far apart are the two nuclei? If they are too far apart, their respective 1s orbitals cannot overlap, and thus no covalent bond can form - they are still just two separate hydrogen atoms. As they move closer and closer together, orbital overlap begins to occur, and a bond begins to form. This lowers the potential energy of the system, as new, attractive positive- negative electrostatic interactions become possible between the nucleus of one atom and the electron of the second . However, something else is happening at the same time: as the atoms get closer, the repulsive positive-positive interaction between the two nuclei also begins to increase. 6
Potential Energy Vs Interatomic Distance At long distances, both attractive and repulsive interactions are small. As the distance between the atoms decreases, the attractive electron proton interactions dominate, and the energy of the system decreases. At the observed bond distance, the repulsive electron electron and proton proton interactions just balance the attractive interactions, preventing a further decrease in the internuclear distance. At very short internuclear distances, the repulsive interactions dominate, making the system less stable than the isolated atoms. Figure 3 7
Potential Energy Vs Interatomic Distance At first this repulsion is more than offset by the attraction between nuclei and electrons, but at a certain point, as the nuclei get even closer, the repulsive forces begin to overcome the attractive forces, and the potential energy of the system rises quickly. When the two nuclei are tooclose , we have an unstable, high-energy situation. There is a defined optimal distance between the nuclei in which the potential energy is at a minimum, meaning that the combined attractive and repulsive forces add up to the greatest overall attractive force. This optimal internuclear distance is the bond length; the distance is 74 pm for H2 9
Potential Energy Vs Interatomic Distance Likewise, the difference in potential energy between the lowest energy state (at the optimal internuclear distance) and the state where the two atoms are completely separated is called the bond dissociation energy, or, more simply, bond strength; the H2 bond strength is 435 kJ/mol. The amount of energy required to break a bond is called bond dissociation energy or simply bond energy. Bond energy is a measure of the strength of a chemical bond. The larger the bond energy, the stronger the bond. 10
Summary Potential Energy Vs Interatomic Distance graph was discussed. At long distances, both attractive and repulsive interactions are small. As the distance between the atoms decreases, the attractive electron proton interactions dominate, and the energy of the system decreases. At the observed bond distance, the repulsive electron electron and proton proton interactions just balance the attractive interactions, preventing a further decrease in the internuclear distance. At very short internuclear distances, the repulsive interactions dominate, making the system less stable than the isolated atoms. 11
