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Polar Bonds and Molecules
Snow covers approximately 23 percent of Earth’s surface. Each individual snowflake is formed from as many as 100 snow crystals. The polar bonds in water molecules influence the distinctive geometry of snowflakes.
8.4
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Polar Bonds and Molecules
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8.4 Bond Polarity
Bond Polarity
How do electronegativity values determine the charge distribution in a polar bond?
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Polar Bonds and Molecules
>8.4 Bond Polarity
When the atoms in a bond pull equally (as occurs when identical atoms are bonded), the bonding electrons are shared equally, and the bond is a nonpolar covalent bond.
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Polar Bonds and Molecules
>8.4 Bond Polarity
The bonding pairs of electrons in covalent bonds are pulled by the nuclei.
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Polar Bonds and Molecules
>8.4 Bond Polarity
The chlorine atom attracts the electron cloud more than the hydrogen atom does.
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Polar Bonds and Molecules
>8.4 Bond Polarity
A polar covalent bond, known also as a polar bond, is a covalent bond between atoms in which the electrons are shared unequally.
The more electronegative atom attracts electrons more strongly and gains a slightly negative charge. The less electronegative atom has a slightly positive charge.
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Polar Bonds and Molecules
> Bond Polarity8.4
Notes from WB: as the difference in electronegativity between the atoms in the bond increases the ionic character of the bond increases and the covalent character decreases.
Ionic bonds are generally formed between metals and nonmetals.
WB gives the range for ionic bonds to be a difference of greater than or equal to 1.7 but you will need to consider the type of elements as well as the electronegativity values. HF, for example , has a polar covalent bond (not an ionic bond).
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Polar Bonds and Molecules
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Polar Molecules
Polar Molecules
What happens to polar molecules between a pair of oppositely charged metal plates?
8.4
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Polar Bonds and Molecules
> Polar Molecules
In a polar molecule, one end of the molecule is slightly negative and the other end is slightly positive.
A molecule that has two poles is called a dipolar molecule, or dipole.
8.4
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Polar Bonds and Molecules
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Polar Molecules
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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Polar Bonds and Molecules
> Polar Molecules
A hydrogen chloride molecule is a dipole.
8.4
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Polar Bonds and Molecules
> Polar Molecules
Animation 10 Learn to distinguish between polar and nonpolar molecules.
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Polar Bonds and Molecules
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8.4 Attractions Between Molecules
Attractions Between Molecules
How do intermolecular attractions compare with ionic and covalent bonds?
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Polar Bonds and Molecules
>8.4 Attractions Between Molecules
Intermolecular attractions are weaker than either ionic or covalent bonds.
These attractions are responsible for determining whether a molecular compound is a gas, a liquid, or a solid at a given temperature.
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Polar Bonds and Molecules
> Attractions Between Molecules
Van der Waals Forces
The two weakest attractions between molecules are collectively called van der Waals forces, named after the Dutch chemist Johannes van der Waals (1837–1923).
8.4
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Polar Bonds and Molecules
> Attractions Between Molecules
Dipole interactions occur when polar molecules are attracted to one another.
8.4
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Polar Bonds and Molecules
>8.4 Attractions Between Molecules
Dispersion forces, the weakest of all molecular interactions, are caused by the motion of electrons.
The strength of dispersion forces generally increases as the number of electrons in a molecule increases.
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Polar Bonds and Molecules
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What are van der waals forces?
http://antoine.frostburg.edu/chem/senese/101/liquids/faq/h-bonding-vs-london-forces.shtml
Hydrogen bonding video:
http://www.youtube.com/watch?v=LGwyBeuVjhU
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Slide 19 of 33
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Polar Bonds and Molecules
> Attractions Between Molecules
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.
8.4
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Polar Bonds and Molecules
>8.4 Attractions Between Molecules
Hydrogen Bonding in Water
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Slide 21 of 33
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Polar Bonds and Molecules
> Attractions Between Molecules
The relatively strong attractive forces between water molecules cause the water to form small drops on a waxy surface.
8.4
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Polar Bonds and Molecules
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Intermolecular Attractions and Molecular Properties
Intermolecular Attractions and Molecular Properties
Why do network solids have high melting points?
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Polar Bonds and Molecules
>8.4 Intermolecular Attractions and Molecular Properties
Network solids (or network crystals) are solids in which all of the atoms are covalently bonded to each other.
Network solids consist of molecules that do not melt until the temperature reaches 1000°C or higher, or they decompose without melting at all.
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Polar Bonds and Molecules
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Intermolecular Properties and Molecular Properties
Melting a network solid would require breaking covalent bonds throughout the solid.
8.4
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Polar Bonds and Molecules
>8.4 Intermolecular Attractions and Molecular Properties
Diamond is an example of a network solid. Diamond does not melt. It vaporizes to a gas at 3500°C or above.
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Polar Bonds and Molecules
>8.4 Intermolecular Attractions and Molecular Properties
Silicon Carbide is a network solid. It has a melting point of about 2700°C.
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Polar Bonds and Molecules
> Intermolecular Attractions and Molecular Properties
Simulation 8 Relate melting and boiling points to the strength of intermolecular forces.
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Polar Bonds and Molecules
> Intermolecular Attractions and Molecular Properties
8.4
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Section Quiz
-or-Continue to: Launch:
Assess students’ understanding of the concepts in Section
8.4 Section Quiz.
8.4.
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8.4 Section Quiz.
1. In a molecule, the atom with the largest electronegativity value
a. repels electrons more strongly and aquires a slightly negative charge.
b. repels electrons more strongly and aquires a slightly positive charge.
c. attracts electrons more strongly and aquires a slightly positive charge.
d. attracts electrons more strongly and aquires a slightly negative charge.
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2. When polar molecules are placed between oppositely charged plates, the negative
a. molecules stick to the positive plates.
b. molecules stick to the negative plates.
c. ends of the molecules turn toward the positive plates.
d. ends of the molecules turn toward the negative plates.
8.4 Section Quiz.
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3. Which of the following bond types is the weakest?
a. ionic bond
b. Van der Waals force
c. covalent bond
d. hydrogen bond
8.4 Section Quiz.