Polarity in Covalent Bonds Bonding Movie

Polar Bond Polar – has polesPolar – has poles –ends are different –bonds: difference in how electrons distributed –Polar bond: one end more electrons than other end

Nonpolar Bond NonNoNonpolar = No poles electron cloud on one end of bond same as other end

H 2 is symmetric: Both ends the same; electron cloud football-shaped HCl is asymmetric: electron cloud lopsided; Cl more electrons than H

Which bond(s) are polar? Which are nonpolar? Polar = LiH & HF Nonpolar = H 2 Red = electron rich Blue = electron poor

How can predict if BOND is polar or nonpolar? Compare electronegativities of two atoms in bond

Electronegativity Ability of atom to attract electrons in bond Found in Table S!

Electronegativities of the Elements

Bond 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 notation

BOND Polarity differenceDepends on electronegativity difference between two atoms in bond A B E A - E B - care only about size of difference, not sign- care only about size of difference, not sign - bigger the difference, the more polar the bond- bigger the difference, the more polar the bond  

Calculate electronegativity difference for each of these bonds LiH : H 2 : HF: 2.1 – – –

Bond Polarity What can say about polarity of bond where atom A = atom B? If both atoms are same element: electronegativity difference = 0

Electronegativity Difference Ionic > 1.7 Polar Covalent >0.5 to 1.7 Nonpolar Covalent 0.0 to 0.5 Type of Bond Electronegativity Difference

Electronegativity & Bond Type

Review Which of following bonds is most polar? –A) O 2 –B) HCl –C) NH in NH 3 –D) HBr 3.4 – 3.4 = – 3.2 = -1.1 = – 2.1 = 0.9 Answer = (B) HCl

Review Which substance contains bond with greatest ionic character? –A) KCl –B) HCl –C) Cl 2 –D) CCl – 3.2 = -2.4 = – 3.2 = -1.1 = – 3.2 = – 3.2 = -0.6 = 0.6 Answer = (A) KCl

Summary Nonpolar covalent bonds form: – between atoms having equal or close electronegativity values: Polar covalent bonds form: –between atoms with electronegativity difference >0.5 to 1.7

Predict the Polarity N 2 HF HCl O 2 Cl 2 HI HBr NonpolarPolar Polar NonpolarNonpolarPolar Polar

MOLECULE Polarity Diatomic molecules: – molecular polarity is same as bond polarity

MOLECULE Polarity (3 + elements) Depends on 2 factors 1. Type bonds in molecule 2. Arrangement of bonds or shape of molecule Think SYMMETRY!!

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

Polarity 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 Molecules Contain at least two mirror planes

Symmetric vs. Asymmetric CO 2 is nonpolar - electron cloud is symmetric H 2 O is polar - electron cloud is lop-sided (asymmetrical)

Polarity of CO 2 ? Draw arrow along each bond pointing to more electronegative atom If arrows cancel out, molecule is NONPOLAR

Polarity of H 2 O? green arrows do NOT cancel out - water is polar! - water is polar!

structural formula can be used to predict Molecular Polarity! H H  C  H H  

Symmetry of Larger Molecules CF 4 is symmetric so is nonpolar

Ethane = C 2 H 6 Ethene = C 2 H 4 Ethyne = C 2 H 2 These molecules are symmetric - electron cloud is same on both ends: they are nonpolar