Polarity in Covalent BondsPolarity in Covalent Bonds

Bonding Movie

Polar BondPolar Bond

• Polar – Polar – has poleshas poles

–ends are differentends are different–bonds: bonds: differencedifference in in how electrons how electrons

distributeddistributed–Polar bond: one end more electrons Polar bond: one end more electrons

than other endthan other end

Nonpolar BondNonpolar Bond

• NonNonpolar = NoNo poles

• electron cloud on one end of bond same as other end

HH22 is symmetric: is symmetric: Both Both ends the same; electron ends the same; electron cloud football-shapedcloud football-shaped

HCl is asymmetric:HCl is asymmetric: electron cloud electron cloud lopsided; Cl more lopsided; Cl more electrons than Helectrons than H

Which bond(s) are polar?Which bond(s) are polar?Which are nonpolar?Which are nonpolar?

Polar = LiH & HF Nonpolar = HPolar = LiH & HF Nonpolar = H22

Red = electron Red = electron richrich

Blue = electron Blue = electron poorpoor

How can predict How can predict if BOND is if BOND is polar or polar or nonpolar?nonpolar?

Compare electronegativities of two Compare electronegativities of two atoms in bondatoms in bond

ElectronegativityElectronegativity

Ability of atom to attract electrons in bond

Found inFound in Table S!Table S!

Electronegativities of the ElementsElectronegativities of the Elements

Bond PolarityBond Polarity

• more electronegative atom pulls bonding electrons more strongly & gets more than fair share of electron cloud

• Leads to separation of charge– electron-rich side is partially negative (-)– electron-poor side is partially positive (+)

Delta notationDelta notation

BOND PolarityBOND Polarity• Depends on electronegativity differencedifference

between two atoms in bond

A BA B EEAA - E - EBB

•- - care only aboutcare only about size of size of difference, not signdifference, not sign

•- - bigger the difference, the more polar the bondbigger the difference, the more polar the bond

Calculate Calculate electronegativity electronegativity difference for each difference for each of these bondsof these bonds

LiH : HLiH : H22 : HF: : HF: 2.1 – 1.02.1 – 1.0 1.11.1

2.1 – 2.12.1 – 2.1 00

2.1 – 42.1 – 4 1.91.9

Bond PolarityBond Polarity

• What can say about polarity of bond where atom A = atom B?

If both atoms are same element: If both atoms are same element: electronegativity difference = 0electronegativity difference = 0

Electronegativity DifferenceElectronegativity Difference

IonicIonic> 1.7> 1.7

Polar CovalentPolar Covalent>>0.5 to 1.70.5 to 1.7

Nonpolar CovalentNonpolar Covalent0.0 to 0.50.0 to 0.5

Type of BondType of BondElectronegativityElectronegativity DifferenceDifference

Electronegativity & Bond TypeElectronegativity & Bond Type

ReviewReview

• Which of following bonds is most polar?–A) O2

–B) HCl –C) NH in NH3

–D) HBr

3.4 – 3.4 = 03.4 – 3.4 = 02.1 – 3.2 = -1.1 = 1.12.1 – 3.2 = -1.1 = 1.1

3.0 – 2.1 = 0.93.0 – 2.1 = 0.93.0 – 2.1 = 0.93.0 – 2.1 = 0.9

Answer = (B) HClAnswer = (B) HCl

ReviewReview

• Which substance contains bond with greatest ionic character?– A) KCl– B) HCl– C) Cl2– D) CCl4

0.8 – 3.2 = -2.4 = 2.40.8 – 3.2 = -2.4 = 2.42.1 – 3.2 = -1.1 = 1.12.1 – 3.2 = -1.1 = 1.1

3.2 – 3.2 = 03.2 – 3.2 = 02.6 – 3.2 = -0.6 = 0.62.6 – 3.2 = -0.6 = 0.6

Answer = (A) KClAnswer = (A) KCl

SummarySummary

• Nonpolar covalent bonds form:– between atoms having equal or close

electronegativity values: 0.0 - 0.4

• Polar covalent bonds form: – between atoms with electronegativity

difference >0.5 to 1.7

Predict the PolarityPredict the Polarity

• N2

• HF• HCl• O2

• Cl2• HI• HBr

NonpolarNonpolarPolarPolarPolarPolar

NonpolarNonpolarNonpolarNonpolarPolarPolarPolarPolar

MOLECULE Polarity

• Diatomic molecules:– molecular polarity is same as bond polarity

MOLECULEMOLECULE Polarity (3 Polarity (3++ elements) elements)

• Depends on 2 factors1. Type bonds in molecule2. Arrangement of bonds or shape of

molecule

Think SYMMETRY!!

For larger molecules:For larger molecules:• look at kind & arrangement of bonds to look at kind & arrangement of bonds to determine overall polarity of moleculedetermine overall polarity of molecule

Polarity of MoleculesPolarity of Molecules

• molecule may contain polar bonds, but not be polar! – depends on geometry of molecule

• If molecule is symmetric:– “pull” of one polar bond is offset by “pull” of

another polar bond– tug-of-war that no one can win!

Symmetric MoleculesSymmetric Molecules

• Contain at least two mirror planes

Symmetric vs. Asymmetric

COCO22 is nonpolar is nonpolar - - electron cloud is symmetricelectron cloud is symmetric

HH22O is polar O is polar -- electron cloud is lop-sided (asymmetrical) electron cloud is lop-sided (asymmetrical)

Polarity of CO2?

Draw arrow along each bond pointing to Draw arrow along each bond pointing to more electronegative atommore electronegative atomIf arrows cancel out, molecule is If arrows cancel out, molecule is NONPOLARNONPOLAR

Polarity of H2O?

green arrows do green arrows do NOT cancel outNOT cancel out - water is polar!- water is polar!

structural formula can be used to predict Molecular Polarity!

HH

H H C C H H HH

Symmetry of Larger MoleculesSymmetry of Larger Molecules

CFCF44 is symmetric is symmetric so is nonpolarso is nonpolar

Ethane = CEthane = C22HH66

Ethene = CEthene = C22HH44

Ethyne = CEthyne = C22HH22

These molecules are symmetric - electron These molecules are symmetric - electron cloud is same on both ends: they are nonpolarcloud is same on both ends: they are nonpolar