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4.9 The Polarity of Covalent Molecules

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

Chapter 4 Forces Between Particles

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Polar Molecules

A polar molecule • contains polar bonds.• has a separation of positive and negative charge

called a dipole indicated with + and -.• has dipoles that do not cancel.

+ - • •

H–Cl H—N—H dipole

H dipoles do not

cancel

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Nonpolar Molecules

A nonpolar molecule • contains nonpolar bonds.

Cl–Cl H–H

• or has a symmetrical arrangement of polar bonds.

O=C=O Cl

Cl–C–Cl

Cl dipoles cancel

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Determining Molecular Polarity

STEP 1: Write the electron-dot formula.STEP 2: Determine the polarity of the bonds.STEP 3: Determine if dipoles cancel.

Example: H2O

. .

H─O: H2O is polar │

H dipoles do not cancel

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Learning Check

Identify each of the following molecules as 1) polar or 2) nonpolar. Explain.

A. PBr3

B. HBrC. Br2

D. SiBr4

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Solution

Identify each of the following molecules as 1) polar or 2) nonpolar. Explain.

A. PBr31) pyramidal; dipoles don’t cancel; polar

B. HBr 1) linear; one polar bond (dipole); polar

C. Br2 2) linear; nonpolar bond; nonpolar

D. SiBr4 2) tetrahedral; dipoles cancel; nonpolar

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The electronegativity value

• indicates the attraction of an atom for shared electrons.

• increases from left to right going across a period on the periodic table.

• is high for the nonmetals with fluorine as the highest.

• is low for the metals.

Electronegativity

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Some Electronegativity Values for Group A Elements

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

Low values

High values

` Electronegativity increases

` E

lectrone

gativity d

ecrease

s

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A nonpolar covalent bond

• occurs between nonmetals.• is an equal or almost equal sharing of electrons.• has almost no electronegativity difference (0.0 to 0.4).

Examples: Electronegativity Atoms Difference Type of BondN-N 3.0 - 3.0 = 0.0 Nonpolar covalent

Cl-Br 3.0 - 2.8 = 0.2 Nonpolar covalentH-Si 2.1 - 1.8 = 0.3 Nonpolar covalent

Nonpolar Covalent Bonds

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A polar covalent bond

• occurs between nonmetal atoms.• is an unequal sharing of electrons.• has a moderate electronegativity difference (0.5 to 1.7).

Examples: Electronegativity

Atoms Difference Type of BondO-Cl 3.5 - 3.0 = 0.5 Polar covalentCl-C 3.0 - 2.5 = 0.5 Polar covalentO-S 3.5 - 2.5 = 1.0 Polar covalent

Polar Covalent Bonds

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Comparing Nonpolar and Polar Covalent Bonds

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Ionic Bonds

An ionic bond • occurs between metal and nonmetal ions.• is a result of electron transfer.• has a large electronegativity difference (1.8 or more).

Examples: Electronegativity

Atoms Difference Type of BondCl-K 3.0 – 0.8 = 2.2 Ionic

N-Na 3.0 – 0.9 = 2.1 Ionic

S-Cs 2.5 – 0.7 = 1.8 Ionic

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TABLE 4.14

Electronegativity and Bond Types

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Predicting Bond Types

Table 4.15

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Use the electronegativity difference to identify the type of bond between the following:nonpolar covalent (NP), polar covalent (P), or

ionic (I).

A. K-NB. N-OC. Cl-ClD. H-Cl

Learning Check

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Use the electronegativity difference to identify the type of bond between the following:nonpolar covalent (NP), polar covalent (P), or

ionic (I).

A. K-N 2.2 ionic (I) B. N-O 0.5 polar covalent (P)

C. Cl-Cl 0.0 nonpolar covalent (NP) D. H-Cl 0.9 polar covalent (P)

Solution

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Other Interparticle Forces

Network Solid – A solid in which the lattice sites are occupied by atoms that covalently bonded to each other.

Dipole-Dipole Interaction – The attractive force that exists between the positive end of one polar molecule and the negative end of another.

Hydrogen Bonding – The result of attractive dipolar forces between molecules in which hydrogen atoms are covalently bonded to very electronegative elements (O, N, or F).

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Dipole alignmentof polar covalent molecules

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Dipole-dipole interactions

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Snowflakes are aresult of intermolecular forces

Credit: Photo Researchers, Inc.

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Ice crystal lattice