Chapter 6 Molecules and Covalent Compounds

6.5

Electronegativity and Bond Polarity

6.6

Polarity of Molecules

Copyright 2005 by Pearson Education, Inc.

Publishing as Benjamin Cummings

Electronegativity

Electronegativity values:

Indicate the attraction of an atom for shared electrons

Increase 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

Some Electronegativity Values for Group A Elements

Copyright 2005 by Pearson Education, Inc.

Publishing as Benjamin Cummings

Low values

High values

Nonpolar Covalent Bonds

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:

Atoms Electronegativity Type of BondDifference

N-N 3.0 - 3.0 = 0.0 Nonpolar covalent

Cl-Br 3.0 - 2.8 = 0.2 Nonpolar covalent

H-Si2.1 - 1.8 = 0.3 Nonpolar covalent

Polar Covalent Bonds

A polar covalent bond,

Occurs between nonmetals atoms

Is an unequal sharing of electrons

Has a moderate electronegativity difference (0.5 to 1.7)

Examples:

Atoms ElectronegativityType of Bond Difference

O-Cl 3.5 - 3.0 = 0.5Polar covalent

Cl-C 3.0 - 2.5 = 0.5Polar covalent

O-S 3.5 - 2.5 = 1.0Polar covalent

Comparing Nonpolar and Polar Covalent Bonds

Copyright 2005 by Pearson Education, Inc.

Publishing as Benjamin Cummings

Ionic Bonds

An ionic bond,

Occurs between metal and nonmetals ions

Is a results of electron transfer

Has a large electronegativity difference (1.8 or more)

Examples:

Atoms ElectronegativityType of Bond Difference

Cl-K 3.0 0.8 = 2.2Ionic

N-Na 3.0 0.9 = 2.1Ionic

S-Cs2.5 0.7= 1.8Ionic

Range of Bond Types

Predicting Bond Types

Learning Check

Use the electronegativity difference to identify the type of bond between the following as:

nonpolar covalent (NP), polar covalent (P), or

ionic (I).

A. K-N

B. N-O

C. Cl-Cl

D. H-Cl

Solution

Use the electronegativity difference to identify the type of bond between the following as:

nonpolar covalent (NP), polar covalent (P), or

ionic (I).

A. K-N2.2ionic (I)

B. N-O0.5 polar covalent (P)

C. Cl-Cl0.0nonpolar covalent (NP)

D. H-Cl0.9polar covalent (P)

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

+ -

HCl ClNCl

dipole

Cl

dipoles do not cancel

Nonpolar Molecules

A nonpolar molecule,

Contains nonpolar bonds

ClCl HH

Or has a symmetrical arrangement of polar bonds

O=C=O Cl

ClCCl

Cl

dipoles cancel

Determining Molecular Polarity

STEP 1 Write the electron-dot formula.

STEP 2 Determine the polarity of the bonds.

STEP 3 Determine if any dipoles cancel or not.

Example: H2O

. .

HO: H2O is polar

H

dipoles do not cancel

Learning Check

Identify each of the following molecules as:

1) polar or 2) nonpolar. Explain.

A. PBr3

B. HBr

C. Br2

D. SiBr4

Solution

Identify each of the following molecules as:

1) polar or 2) nonpolar. Explain.

A. PBr31) pyramidal; dipoles dont cancel; polar

B. HBr1) linear; one polar bond (dipole); polar

C. Br22) linear; nonpolar bond; nonpolar

D. SiBr42) tetrahedral; dipoles cancel; nonpolar