Polarity in Covalent BondsPolarity in Covalent Bonds

Polar BondPolar Bond

• polar polar means having means having polespoles• uneven distribution uneven distribution of electrons of electrons

• one end of bond has more electrons than other endone end of bond has more electrons than other end

• large electronegativity difference large electronegativity difference between bonded between bonded atomsatoms

Non-polar BondNon-polar Bond

• non-non-polar = nono poles• even distribution of electrons

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

• low or no electronegativity difference between bonded atoms

HH22 is symmetric: is symmetric: - both ends the same - both ends the same - electron cloud is football-shaped- electron cloud is football-shaped

HCl is asymmetric:HCl is asymmetric: - both ends are different- both ends are different- electron cloud is lopsided - electron cloud is lopsided

- - (Cl side more e(Cl side more e-- than H side) than H side)

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

polar: LiH & HF non-polar: Hpolar: LiH & HF non-polar: H22

red: ered: e-- rich rich

blue: eblue: e-- poor poor

How can predict How can predict if if BONDBOND is is polar or not?polar or not?

compare electronegativity values of compare electronegativity values of two atoms in bondtwo atoms in bond

ElectronegativityElectronegativity

ability of atom to attract electrons in bond

values found invalues found in Table S!Table S!

Bond PolarityBond Polarity• more electronegative atom pulls bonding e-

more strongly & gets more than fair share of electron cloud– leads to separation of charge (delta notation)

• electron-rich side is partially negative (-)• electron-poor side is partially positive (+)

BONDBOND Polarity Polarity

• depends on electronegativity differencedifference between two atoms (A and B) in bond

A BA B EEAA - E - EBB

•- - care only about care only about SIZESIZE of difference, of difference, not signnot sign

•- - biggerbigger the the difference, difference, thethe more polar more polar thethe bond bond

Bond PolarityBond Polarity

• What can say about polarity of bond where atom A is same as atom B?

both atoms are same so both atoms are same so electronegativity difference = 0electronegativity difference = 0

therefore bond will be therefore bond will be non-polarnon-polar

Electronegativity DifferenceElectronegativity Difference

ionicionic> 1.7> 1.7

polar covalentpolar covalent>>0.5 to 1.70.5 to 1.7

non-polar covalentnon-polar covalent0.0 to 0.50.0 to 0.5

Type of Type of

BondBondDifference in Difference in

ElectronegativitiesElectronegativities

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

polar covalent non- polar covalent polar covalent

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 = 3.4 – 3.4 = 002.1 – 3.2 = -1.1 = 2.1 – 3.2 = -1.1 = 1.11.1

3.0 – 2.1 = 3.0 – 2.1 = 0.90.93.0 – 2.1 = 3.0 – 2.1 = 0.90.9

answer =(B) answer =(B) HClHCl

ReviewReview

• Which substance contains bond with greatest ionic character?A) KCl

B) HCl

C) Cl2D) CCl4

0.8 – 3.2 = -2.4 0.8 – 3.2 = -2.4 = = 2.42.42.1 – 3.2 = -1.1 = 2.1 – 3.2 = -1.1 = 1.11.1

3.2 – 3.2 =3.2 – 3.2 = 00

2.6 – 3.2 = -0.6 = 2.6 – 3.2 = -0.6 = 0.60.6

answer = (A) answer = (A) KClKCl

SummarySummary

• non-polar covalent bonds form between:– atoms having equal or low electronegativity

values: 0.0 - 0.5

• polar covalent bonds form between: – atoms with greater electronegativity difference

in values: >0.5 to 1.7

Predict the PolarityPredict the Polarity

• N2

• HF

• HCl

• O2

• Cl2• HI

• HBr

non-polarnon-polar

polarpolar

polarpolarnon-polarnon-polar

non-polarnon-polar

polarpolar

polarpolar

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 molecule

2. 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 of symmetry

diatomic elements

CO2

CH4

Symmetric vs. Asymmetric

COCO22 is nonpolar: is nonpolar: electron cloud is symmetricelectron cloud is symmetric

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

Polarity of CO2?

two planes of symmetry therefore, molecule is two planes of symmetry therefore, molecule is NONPOLARNONPOLAR

Polarity of H2O?

only one plane of symmetry not two, therefore only one plane of symmetry not two, therefore water is water is polarpolar!!

structural formula can be used to predict Molecular Polarity!

HH

H H C C H H HH

CFCF44 is symmetric is symmetric so is nonpolarso is nonpolar

Ethane = CEthane = C22HH66

Ethene = CEthene = C22HH44

Ethyne = CEthyne = C22HH22

ALL hydrocarbon molecules are symmetrical ALL hydrocarbon molecules are symmetrical - electron cloud is same on both ends - electron cloud is same on both ends - they are non-polar- they are non-polar