Section 8.5 Electronegativity and Polarity

• Describe how electronegativity is used to determine bond type.

electronegativity: the relative ability of an atom to attract electrons in a chemical bond

• Compare and contrast polar and nonpolar covalent bonds and polar and nonpolar molecules.

• Generalize about the characteristics of covalently bonded compounds.

Section 8.5 Electronegativity and Polarity (cont.)

polar covalent bond

A chemical bond’s character is related to each atom’s attraction for the electrons in the bond.

Electron Affinity, Electronegativity, and Bond Character

•Most bonds are a blend of ionic and Most bonds are a blend of ionic and covalent characteristics.covalent characteristics.

•Difference in Difference in electronegativityelectronegativity determines bond type.determines bond type.

ElectronegativityElectronegativityAttraction an atom has for a shared Attraction an atom has for a shared pair of electrons.pair of electrons.higher e-neg atom higher e-neg atom --lower e-neg atomlower e-neg atom ++

Electron Affinity, Electronegativity, and Bond Character (cont.)

• This table lists the character and type of chemical bond that forms with differences in electronegativity.

Electron Affinity, Electronegativity, and Bond Character (cont.)

• Unequal sharing of electrons results in a polar covalent bond.

• Bonding is often not clearly ionic or covalent.• This graph summarizes the range of

chemical bonds between two atoms.

Polar Covalent Bonds

• Polar covalent bonds form when atoms pull on electrons in a molecule unequally.

• Electrons spend more time around one atom than another resulting in partial charges at the ends of the bond called a dipole.

Examples:

• Cl2

• HCl

• NaCl

3.0-3.0=0.0Nonpolar

3.0-2.1=0.9Polar

3.0-0.9=2.1Ionic

Nonpolar Covalent BondNonpolar Covalent Bond ee-- are are sharedshared equallyequally symmetrical esymmetrical e-- density density usually identical atoms usually identical atoms

Bond PolarityBond Polarity

+ -

Bond PolarityBond Polarity

Polar Covalent BondPolar Covalent Bond ee-- are are shared unequallyshared unequally asymmetrical easymmetrical e-- density density results in results in partialpartial charges (dipole) charges (dipole)

NonpolarNonpolar

PolarPolar

IonicIonic

Bond PolarityBond Polarity

Dipole MomentDipole Moment

Direction of the polar bond in a molecule.Direction of the polar bond in a molecule.

Arrow points toward the more eArrow points toward the more e--neg atom.neg atom.

H Cl+ -

Determining Molecular PolarityDetermining Molecular Polarity

Depends onDepends on:: dipole momentsdipole moments molecular shapemolecular shape

Determining Molecular PolarityDetermining Molecular Polarity

Nonpolar MoleculesNonpolar Molecules Dipole moments are symmetrical and Dipole moments are symmetrical and

cancel out.cancel out. Or…Or… Bonds are nonpolarBonds are nonpolar

BF3

F

F F

B

Determining Molecular PolarityDetermining Molecular Polarity

Polar MoleculesPolar Molecules Must have polar bonds and…Must have polar bonds and… Dipole moments are asymmetrical and don’t Dipole moments are asymmetrical and don’t

cancel .cancel .

netdipolemoment

H2OH H

O

CHCl3

H

Cl ClCl

Determining Molecular PolarityDetermining Molecular Polarity

Therefore, polar molecules have...Therefore, polar molecules have... asymmetrical shape (lone pairs) or asymmetrical shape (lone pairs) or asymmetrical atomsasymmetrical atoms

netdipolemoment

Intermolecular ForcesIntermolecular ForcesAttractions are weak compared to ionic and Attractions are weak compared to ionic and covalent bonds between atoms covalent bonds between atoms (intra-bonding) = strong(intra-bonding) = strong

Forces of attraction between two molecules Forces of attraction between two molecules (inter-bonding) = weak(inter-bonding) = weak

Three Types of IMF…Three Types of IMF… Hydrogen BondingHydrogen Bonding – Strongest of the weak – Strongest of the weak DipoleDipole (polar) Bonding – medium of the (polar) Bonding – medium of the

weakweak Dispersion ForcesDispersion Forces – weakest of the weak – weakest of the weak

Hydrogen Hydrogen BondingBonding

Bonding between hydrogen and Second row elements N, O, & F only.

http://chemmovies.unl.edu/ChemAnime/index.htm

Dipole-Dipole Dipole-Dipole AttractionsAttractions

•Attractions between polar neighboring molecules. •Not as strong as H-bonding

Dispersion ForcesDispersion Forces•Are caused by the Are caused by the movements of electrons movements of electrons around the nucleus of an around the nucleus of an atom. atom. (Nonpolar (Nonpolar Molecules only) Molecules only) •More electrons you have More electrons you have the stronger the DF.the stronger the DF.

Fritz LondonFritz London1900-19541900-1954 Dispersion forces Dispersion forces

increase with the size of increase with the size of the molecules. (the molecules. (number number of atomsof atoms))

Relative magnitudes of forcesRelative magnitudes of forces

Ionic & Covalent bonds (between atoms)

Hydrogen bonding (between molecules)

Dipole-dipole interactions (between molecules)Dispersion Forces (between molecules)

Strongest

Weakest

A. A

B. B

C. C

D. D

Section 8-5

A B C D

0% 0%0%0%

Section 8.5 Assessment

The force between water molecules is what kind of intermolecular force?

A. induced dipole

B. hydrogen bond

C. sigma bond

D. partial dipole

A. A

B. B

C. C

D. D

Section 8-5

Section 8.5 Assessment

A B C D

0% 0%0%0%

What kind of bond occurs within a molecule with unequal sharing of electron pairs?

A. ionic bond

B. sigma bond

C. non-polar covalent bond

D. polar covalent bond