chapter 8 section 4 notes

1. Section 8.4
Polar Bonds and Molecules
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2. Bond Polarity
Covalent bonds involve electron sharing between atoms.
However, they differ in terms of how the bonded atoms
share the electrons.
The bonding pairs of electrons
are pulled between the nuclei
of the atoms sharing the e-
When the atoms in the bond pull
Equally (identical atoms are bonded), the bonding
electrons are shared equally and the bond is a nonpolar
covalent bond.
H2, O2, N2, and Cl2 all have nopolar covalent bonds
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3. Bond Polarity
A polar covalent bond (polar bond), is a covalent bond
between atoms in which the electrons are shared
unequally.
The more electronegative atoms attracts electrons more
strongly and gains a slightly negative charge.
The less electronegative atoms has a slightly positive
charge.
Fluorine is the most electronegative and oxygen is the
second most electronegative element.
Electronegativity describes the attraction an atom has
for electrons when the atom is in a compound.
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4. 4

5. Electronegativity Values of Some Elements
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6. Bond Polarity
The electronegativity difference between two atoms tells
you what kind of bond is likely to form.
As the electronegativity difference between two atoms
increases, the polarity of the bond increases.
If the electronegativity difference is greater than 2.0, it is
very likely that electrons will be pulled away completely
by one of the atoms. (an ionic bond will form)
EN Differences Probable Bond Type Example
0.0 – 0.4 Non polar covalent H – H (0.0)
0.5 – 0.99 Moderately polar covalent H – Br (0.8)
1.0 – 2.0 Very polar covalent H – F (1.8)
> 2.0 ionic Na – Cl (2.27)6

7. Bond Polarity
In hydrogen chloride (HCl) molecule, the H has an
electronegativity of 2.2 and the chlorine has an
electronegativity of 3.2.
The values are significantly different, so the covalent
bond in HCl is polar.
The chlorine atom acquires a slightly negative charge
and the hydrogen atoms acquires a slightly positive
charge.
Greek letter delta () indicated that the atoms involved in
the covalent bond acquire only partial charges, less than
1+ or 1-. + - + - + + -
H – Cl H – O – H H - Br 7

8. Problems
Place the following covalent bonds in order from least to
most polar. a. H – Cl b. H – Br c. H – S d. H - C
c & d (tie), b, a
Identify the bonds between atoms of each pair of elements
as nonpolar covalent, moderately polar covalent, very
polar covalent, or ionic.
a. H & Br b. K & Cl c. C & O d. Cl & F e. Li & O
f. Br & Br
a. moderately polar covalent b. ionic c. nonpolar
covalent d. nonpolar covalent e. ionic f. nonpolar
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9. Polar Molecules
The presence of a polar bond in a molecule often makes
the entire molecule polar.
In a polar molecule, one end of the molecule is slightly
negative and the other end is slightly positive.
In HCl, the partial charges on the hydrogen and chlorine
atoms are electrically charged regions or poles.
A molecule that has two poles is called a dipolar
molecule.
When polar molecules are placed between oppositely
charged plates, they tend to become oriented with
respect to the positive and negative plates.
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10. Polar Molecules
CO2 has two polar bonds and is linear. The C and O lies
along the same axis, thus the bond polarities cancel
because they are in opposite directions.
CO2 is a nonpolar molecule despite the presence of two
polar bonds.
Water also has two polar bonds. The water molecule is
bent and the bond polarities do not
cancel and the water molecule is polar.
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11. Attractions Between Molecules
Molecules can attract each other by a variety of forces.
Intermolecular attractions are weaker than either ionic or
covalent bonds.
Intermolecular forces are responsible for determining
whether a molecular compound is a gas, a liquid, or a
solid at a given temperature.
Van der Waals Forces – are the two weakest attractions
between molecules and include both:
• dipole interactions
• dispersion forces
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12. Dipole Interactions
Dipole interactions occur when polar molecules are
attracted to one another. The attraction involved occurs
between the oppositely charged regions of polar
molecules.
The slightly negative region is weakly attracted to the
slightly positive regions of another molecule.
Dipole interactions are similar to but much weaker than
ionic bonds.
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13. Dipole Interactions
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14. Dispersions Forces
Dispersion forces are the weakest of all molecular
interactions and are caused by the motion of electrons.
They occur between nonpolar molecules.
When the moving electrons happen to be momentarily
more on the side of a molecule closest to a neighboring
molecule, their electric force influences the neighboring
molecule’s electrons to be momentarily more on the
opposite side.
This causes an attraction between the two molecules
similar but much weaker than the attraction between
polar molecules.
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15. Dispersions Forces
The strength of dispersion forces generally increases as
the number of electrons in a molecule increases.
The halogen diatomic molecules attract each other
mainly by means of dispersion forces.
Fluorine and chlorine has relatively few electrons and
are gases at room temperature because of their weak
dispersion forces.
Bromine has a larger number of electrons and generates
larger dispersion forces. Bromine molecules attract each
other sufficiently to make it a liquid at room temperature.
Iodine, with a larger number of electrons, is a solid at
room temperature. 15

16. Dispersions Forces
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17. Hydrogen Bonds
The dipole interactions in water produce an attraction
between water molecules.
Each O – H bond in the water molecule is very polar and
the O acquires a slightly negative charge.
The positive region of one water molecules attracts the
negative region of another water molecule.
This attraction between the hydrogen of one water
molecule and the oxygen of another water molecule is
strong compared to other dipole interaction.
This strong attraction, (also found in other H containing
molecules) is called hydrogen bonding.
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18. 18

19. Hydrogen Bonds
Hydrogen bonds are attractive forces in which a
hydrogen covalently bonded to a very electronegative
atom is also weakly bonded to an unshared electron pair
of another electronegative atom.
The other atom may be in the same molecule or in a
nearby molecule.
Hydrogen bonding always involves hydrogen.
The bond between hydrogen and a very electronegative
atom is strongly polar.
Hydrogen bonds are the strongest of the intermolecular
forces.
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20. End of Chapter 8
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