It is the 2nd part of the presentation of slides on Chemical Bonding.

1. Continued from Chemical
Bonding-1 by Aditya Abeysinghe
For more information about the
basics of bonding please visit the
above mentioned presentation
Chemical Bonding-2 by Aditya
Abeysinghe
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2. Chemical Bonding-2 by Aditya
Abeysinghe
Chemical Bonding-2 by Aditya
Abeysinghe
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3. Polarity
Polarity is the measurement which depicts the
tendency of an atom’s, of a molecule’s or of an ion’s
to separate into poles.
(Here poles are the two ends of the bond in which
molecules are bonded or in which atoms are self
polarized as the positive and the negative poles.)
There are two factors that affect the polarity of a
substance:
1. The size of the substance
2. The distribution of the electrons around the
substance
Chemical Bonding-2 by Aditya
Abeysinghe
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4. 1. Size of the substance-
When the size of the substance increases, it is more
tendentious to be separated into poles. Thus the polarity of
such a substance is higher than that of a smaller substance.
(When the size of a substance increases, the electrons in the
outermost shell show less attraction to the nucleus than in a
smaller atom)
2. The distribution of electrons-
When there’s an asymmetric electron distribution
around the nucleus of a substance its polarity is high as
compared to that in a substance with a symmetrical electron
distribution (Here asymmetric means that the resultant of the
forces directed towards the nucleus of a substance is not zero)
E.g.: Ammonia is asymmetrical due to the presence of a lone
pair of electron whereas methane is symmetrical as the
resultant forces of any 3 bonds will be balanced by the fourth
force to make the molecule symmetrical.
Chemical Bonding-2 by Aditya
Abeysinghe
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5. Dipole Moment (μ)
Dipole moment is the product of the charge of a pole
and the distance between the two charges.
The SI unit of the dipole moment is the coulomb meter.
However, since the measurements of these types are so
small, the dipole moment is usually calculated by a unit
called Debye
1 Debye (D) = 3.34 × 10-30 Cm . (Here Cm means
Coulomb meter)
Dipole moment is a key factor in deciding about the
variation of the intermolecular attractions.
Dipole moments of some compounds are as follows:
Chemical Bonding-2 by Aditya
Abeysinghe
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6. Chemical Bonding-2 by Aditya
Abeysinghe
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Molecule Dipole Moment (D)
H2 0
HF 1.78
HCl 1.07
HBr 0.79
HI 0.38
ClF 0.88
BrF 1.29
BrCl 0.52
H2O 1.85
H2S 0.95
CO2 0
NH3 1.47
NF3 0.23
NCl3 0.39
CH4 0

7. Intermolecular forces of attraction
The term intermolecular refers to attractions
between molecules.
There are different types of such attractions:
1. Dipole-Dipole forces
2. Dipole-ion forces
3. Ion-induced dipole forces
4. Dipole-induced dipole forces
5. Induced dipole-Induced dipole forces
6. Hydrogen bonds
7. Vanderwall’s forces
Chemical Bonding-2 by Aditya
Abeysinghe
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8. Dipole-dipole forces
The interactions between atoms through their
oppositely charged poles are called dipole-dipole
forces.
Chemical Bonding-2 by Aditya
Abeysinghe
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δ+ δ-
δ + represents the positive pole of the atom
while δ - represents the negative pole of the
atom
Broken lines showing the
intermolecular attractions-
dipole-dipole forces
Due to this bond single elements
cannot escape from the solution in
which they exist. Thus, melting
points of such substances are high.
These attractive forces are weaker
than the full charges carried by ions
in ionic crystals

9. Dipole-ion forces
The forces between cations and anions due to the
presence of an electric field between them can be
summarized as follows:
Chemical Bonding-2 by Aditya
Abeysinghe
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+ -

10. These bonds are important in forming hydrates
and ammoniates of compounds.
E.g.: [Mg.6H2O]2+ , [Ca.6NH3]2+
Chemical Bonding-2 by Aditya
Abeysinghe
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11. Ion-induced dipole forces
A nonpolar molecule or atom may be polarized by
an electron cloud of an ion and the induced dipole
may make a bond with the ion. Such a bond is
called an ion-induced dipole force.
Chemical Bonding-2 by Aditya
Abeysinghe
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δ+
I I
I-
Iodide
ion Iodine (I2 )
molecule
I3
- Compound
Another example- Li+ nG , where G is a noble gas and n= 1 or 2

12. Dipole- Induced dipole bond
An atom or a molecule which is nonpolar may be
induced by a field of a permanent dipole and the
formation of a bond between the permanent
dipole and the induced dipole is called a dipole-
induced dipole bond.
E.g.: Noble gas hydrates (G.xH2O ; maximum x=6)
Chemical Bonding-2 by Aditya
Abeysinghe
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H
O -2δ
Dipole
(Water)
Electrically neutral
substance later
induced and
separated into poles
by the inducing
action of
water(dipole)
Induced dipole

13. Induced dipole-Induced dipole bonds
Although the electron cloud of a noble gas may be
symmetrically arranged, it can be asymmetrical quickly
due to the movement of electrons.
Therefore, noble gases and nonpolar molecules act as
temporary induced dipoles. Self-induced dipoles polarize
more non-polar atoms or molecules. Such a bond is called
an induced dipole-induced dipole bond.
Chemical Bonding-2 by Aditya
Abeysinghe
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Dipole
Induced dipole-1 Induced dipole- 2
Induced
dipole-
Induced
dipole
bond

14. London forces
London forces are a type of force that can be found
in both polar and nonpolar molecules.
A nonpolar atom/ molecule may sometimes
become polar due to the constant motion of its
electrons. This might even cause uneven charge
distribution.
When this occurs, the molecule/atom has a
temporary dipole.
This dipole can then cause a second atom be
attached to the opposite pole of the first molecule.
Chemical Bonding-2 by Aditya
Abeysinghe
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15. London dispersion forces are the weakest of all the electronic forces
that act between atoms or between molecules.
Due to these forces nonpolar substances such as noble gases and
halogens condense into liquids and then freeze into solids when the
temperature is lowered sufficiently
London force are the cause as to why liquids are composed of
molecules with no permanent dipole attraction and have low boiling
points compared to their molecular masses.
Solids which have London dispersion attraction have rather soft
crystals, are easily deformed, and vaporize easily.
Because of the low intermolecular forces, the melting points are also
low and evaporation takes place so easily that it may occur at room
temperature
E.g.: Iodine crystals , Moth Balls (Para dichlorobenzene and
naphthalene ).
Chemical Bonding-2 by Aditya
Abeysinghe
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16. Hydrogen Bonds
A proton or hydrogen nucleus has a high
concentration of positive charge when a
hydrogen atom is bonded to a highly
electronegative atom. Its positive charge will
have an attraction for neighboring electron pairs.
This special kind of dipole attraction is called
hydrogen bond.
Chemical Bonding-2 by Aditya
Abeysinghe
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17. Chemical Bonding-2 by Aditya
Abeysinghe
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C
Cl
H
CH3
O
Due to the presence of hydrogen bonds, there’s an increase in the melting and boiling
points of elements where hydrogen bonds are present
By energy of bonds,
London Forces < Hydrogen bonds < Primary bonds (ionic,
metallic, and covalent bonds)

18. Double and triple bonds
To achieve the octet structure some atoms to
share two or even three pairs of electrons.
Sharing two pairs- Double bond (CO2 )
Sharing three pairs- Triple bond (C2H2)
In double and triple bonds the attractive force
between the nuclei and the shared electrons is
greater as compared to the bonds where single
bonds are formed.
Chemical Bonding-2 by Aditya
Abeysinghe
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19. ΔHD (Single Bond) < ΔHD (Double bond) < ΔHD (Triple bond)
Heat needed to break the bonds increases
Because double bonds and triple bonds tend to
pull atoms close together,
Single bond > Double bond > Triple bond
Intermolecular distance increases
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Abeysinghe
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20. Resonance Structures
It’s not always possible to represent the bonding
structure of a molecule by either the Lewis dot
structure or the line drawing structure.
This is because data about the bond strengths and
bond distance reveal that most molecules have a
hybrid(resonant) structure.
Thus many molecules are represented as a series of
transitions of bond structures, not as an individual
component.
E.g.: Sulfur trioxide and Benzene.
Chemical Bonding-2 by Aditya
Abeysinghe
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21. Chemical Bonding-2 by Aditya
Abeysinghe
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S
O
Resonance
Structure 1
Resonance
Structure 2
Resonance
Structure 3
O
S
This intermediate form between
different contributing structures is
called a resonance
hybrid(true structure)

22. The actual or the true structure is always stable than the resonance structure
and the energy of the actual structure is thus lower than the resonance
structures
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Abeysinghe
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ΔERE – Resonance
energy difference
ΔH /
KJmol-1
(Resonance
Structures)
(Resonance Hybrid
(True Structure))

23. Although sulfur trioxide is an overall neutral molecule,
many ions too show resonance.
The charge distribution of such an actual structure is as
follows:
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Abeysinghe
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- 2/3
*The three -2/3 charges on the three oxygen atoms add up to the total of -2 charge on the
overall molecule.