bond polarity 8.4 polar bonds and molecules > bond...
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8.4 Polar Bonds and Molecules >
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Describing Polar Covalent Bonds Electron density of a hydrochloric acid molecule
Bond Polarity 8.4 Polar Bonds and Molecules >
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Describing Polar Covalent Bonds
Polarity may also be represented by an arrow pointing to the more electronegative atom.
Bond Polarity
H—Cl
8.4 Polar Bonds and Molecules >
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Bond Polarity
Describing Polar Covalent Bonds
The O—H bonds in a water molecule polar.
8.4 Polar Bonds and Molecules >
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Bond Polarity
Electronegativity Differences and Bond Types
Electronegativity difference range
Most probable type of bond Example
0.0–0.4 Nonpolar covalent H—H (0.0)
0.4–1.0 Moderately polar covalent δ+ δ– H—Cl (0.9)
1.0–2.0 Very polar covalent δ+ δ– H—F (1.9)
>2.0 Ionic Na+Cl– (2.1)
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Bond Polarity
Describing Polar Covalent Bonds There is no sharp boundary between ionic and covalent bonds. • As the electronegativity difference between
two atoms increases, the polarity of the bond increases.
• If the difference is > 2.0, the electrons will likely be pulled away completely by one of the atoms (ionic bond).
8.4 Polar Bonds and Molecules >
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Sample Problem 8.3
Identifying Bond Type Which type of bond (nonpolar covalent, moderately polar covalent, very polar covalent, or ionic) will form between each of the following pairs of atoms?
a. N and H
b. F and F
c. Ca and Cl
d. Al and Cl
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Sample Problem 8.3
Identify the electronegativities of each atom using Table 6.2.
a. N(3.0), H(2.1)
b. F(4.0), F(4.0)
c. Ca(1.0), Cl(3.0)
d. Al(1.5), Cl(3.0)
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Sample Problem 8.3
a. N(3.0), H(2.1); 0.9; moderately polar covalent
b. F(4.0), F(4.0); 0.0; nonpolar covalent
c. Ca(1.0), Cl(3.0); 2.0; ionic
d. Al(1.5), Cl(3.0); 1.5; very polar covalent
Calculate the electronegativity difference between the two atoms.
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8.4 Polar Bonds and Molecules >
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• In a polar molecule, one end of the molecule is slightly negative, and the other end is slightly positive.
Bond Polarity
Describing Polar Covalent Molecules The presence of a polar bond in a molecule often makes the entire molecule polar.
8.4 Polar Bonds and Molecules >
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• A molecule that has two poles is called a dipolar molecule, or dipole.
Bond Polarity
Describing Polar Covalent Molecules δ+ δ– H—Cl
8.4 Polar Bonds and Molecules >
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Bond Polarity
Describing Polar Covalent Molecules
8.4 Polar Bonds and Molecules >
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• A carbon dioxide molecule has two polar bonds and is linear.
O C O
Bond Polarity
Describing Polar Covalent Molecules
Polar bonds Nonpolar Molecule
Polar bonds in the same plane pointing in opposing directions
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The water molecule also has two polar bonds.
Bond Polarity
Describing Polar Covalent Molecules
• But, the bond polarities do not cancel
• So the molecule is polar.
Polar Bonds, Polar Molecule
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• Is there a high electronegative element (N, O, F, Cl) bonded to a low electronegative element (C, H, P)?
• Do the bond polarities all point toward one end of the molecule?
• Does the central atom have lone pair(s)?
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Predicting Polarity
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• Polar or Nonpolar? • F2 (or any of the diatomics, for that
matter) • Carbon tetrachloride • Phosphorus pentachloride • Boron trifluoride • Ammonia (NH3)
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Describing Polar Covalent Molecules
8.4 Polar Bonds and Molecules >
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Attractions Between Molecules
• How do the strengths of intermolecular attractions compare with ionic and covalent bonds?
Attractions Between Molecules
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• Intermolecular (interparticle) attractions are weaker than either ionic or covalent bonds.
• Inter = between
Attractions Between Molecules
Molecules can be attracted to each other by a variety of different forces.
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Van der Waals Forces
Two types of attractions between molecules • dipole interactions (strongest of the
weak)
• dispersion forces (weakest of the weak)
Attractions Between Molecules
8.4 Polar Bonds and Molecules >
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Van der Waals Forces
Dipole interactions occur between polar molecules
Attractions Between Molecules
• electrical attraction between the oppositely charged regions of polar molecules (dipoles).
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Attractions Between Molecules
Van der Waals Forces • Dipole interactions
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Van der Waals Forces
Dispersion forces
• the weakest of all molecular interactions
• caused by the motion of electrons.
Attractions Between Molecules
• between nonpolar molecules.
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Van der Waals Forces
Dispersion forces
Attractions Between Molecules
• E- momentarily on one side of a molecule
• Electric force temporarily repels e- in a neighboring molecule.
• The strength of dispersion forces generally increases as the number of electrons in a molecule increases.
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Van der Waals Forces
Attractions Between Molecules
• Fluorine and chlorine have relatively few electrons and are gases at ordinary room temperature and pressure because of their especially weak dispersion forces.
• Bromine molecules therefore attract each other sufficiently to make bromine a liquid under ordinary room temperature and pressure.
• Iodine, with a still larger number of electrons, is a solid at ordinary room temperature and pressure.
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Hydrogen Bonds
Attractions Between Molecules
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Hydrogen Bonds
Attractions Between Molecules
Hydrogen bonds
• Special case of dipole interactions
• Hydrogen in one molecule
• N, O, or F in a neighboring molecule
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Hydrogen Bonds
Attractions Between Molecules
95 % weaker than the average covalent bond.
• Strongest of the intermolecular forces.
• Extremely important in determining the properties of water and biological molecules.
8.4 Polar Bonds and Molecules >
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A snowflake’s shape is determined by the interactions of hydrogen bonds during its formation.
CHEMISTRY & YOU 8.4 Polar Bonds and Molecules >
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Intermolecular Attractions and Molecular Properties
• Why are the properties of covalent compounds so diverse?
Intermolecular Attractions and Molecular Properties
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Intermolecular Attractions and Molecular Properties
The physical properties of a compound depend on the type of bonding it displays—in particular, on whether it is ionic or covalent.
state at room temperature
melting point, boiling point
The diversity of physical properties among covalent compounds is mainly because of widely varying intermolecular attractions.
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• Intermolecular forces are much weaker than ionic and covalent bonds
Intermolecular Attractions and Molecular Properties
The melting and boiling points of most molecular compounds are low (compared with those of ionic compounds).
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• Most of these very stable substances are network solids (or network crystals)
Intermolecular Attractions and Molecular Properties
A few solids that consist of molecules do not melt until the temperature reaches 1000°C or higher.
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Intermolecular Attractions and Molecular Properties
Diamond is an example of a network solid. • Each carbon atom in a
diamond is covalently bonded to four other carbons, interconnecting carbon atoms throughout the diamond.
• Diamond does not melt; rather, it vaporizes to a gas at 3500°C and above.
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Characteristics of Ionic and Molecular Compounds Characteristic Ionic Compound Molecular Compound
Representative unit Formula unit Molecule
Bond formation Transfer of one or more electrons between atoms
Sharing of electron parts between atoms
Type of elements Metallic and nonmetallic Nonmetallic
Physical state Solid Solid, liquid, or gas
Melting point High (usually above 300°C) High (usually below 300°C)
Solubility in water Usually high Usually low
Electrical conductivity of aqueous solution Good conductor Poor to non-conducting
Intermolecular Attractions and Molecular Properties
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Why do network solids take so much more heat to melt than most covalent compounds? Melting a network solid requires breaking covalent bonds throughout the solid. Melting most covalent compounds only requires breaking the weak attractions between molecules.