Polarity in Covalent Bonds Bonding Movie

Polar Bond Polar means poles uneven distribution of electrons Polar bond: one end of bond has more electrons than other end

Nonpolar Bond Nonpolar = No poles even distribution of electrons electron cloud same on one end of bond as other end

H2 is symmetric: Both ends the same; electron cloud football-shaped HCl is asymmetric: electron cloud lopsided; Cl side more electrons than H side

Which bond(s) are polar? Which are nonpolar? Polar: LiH & HF Nonpolar: H2 Red: e- rich Blue: e- poor

How can predict if BOND is polar or not? Compare electronegativity values 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 e- 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 A B EA - EB   Depends on electronegativity difference between two atoms in bond A B EA - EB   - care only about size of difference, not sign - bigger the difference, the more polar the bond

Calculate electronegativity difference for each of these bonds 2.1 – 4 1.9 LiH : H2 : HF: 2.1 – 1.0 1.1 2.1 – 2.1

Bond Polarity What can say about polarity of bond where atom A is same as atom B? If both atoms are same element: electronegativity difference = 0 therefore will be non-polar

Electronegativity Difference ionic > 1.7 polar covalent >0.5 to 1.7 nonpolar covalent 0.0 to 0.5 Type of Bond Difference in Electronegativities

Electronegativity & Bond Type

Review Which of following bonds is most polar? A) O2 B) HCl C) NH in NH3 D) HBr 3.4 – 3.4 = 0 2.1 – 3.2 = -1.1 = 1.1 3.0 – 2.1 = 0.9 3.0 – 2.1 = 0.9 Answer =(B) HCl

Review Which substance contains bond with greatest ionic character? A) KCl B) HCl C) Cl2 D) CCl4 0.8 – 3.2 = -2.4 = 2.4 2.1 – 3.2 = -1.1 = 1.1 3.2 – 3.2 = 0 2.6 – 3.2 = -0.6 = 0.6 Answer = (A) KCl

Summary Nonpolar covalent bonds form: Polar covalent bonds form: between atoms having equal or close electronegativity values: 0.0 - 0.5 Polar covalent bonds form: between atoms with electronegativity difference >0.5 to 1.7

Predict the Polarity Nonpolar N2 HF HCl Polar O2 Cl2 Polar HI HBr

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

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 of symmetry

Symmetric vs. Asymmetric CO2 is nonpolar: electron cloud is symmetric H2O is polar: electron cloud lop-sided (asymmetrical)

Polarity of CO2? Draw arrow along each bond pointing to more electronegative atom If arrows cancel out, molecule is NONPOLAR

Polarity of H2O? green arrows do NOT cancel out - water is polar!

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

Symmetry of Larger Molecules CF4 is symmetric so is nonpolar

Ethane = C2H6 Ethene = C2H4 Ethyne = C2H2 These molecules are symmetric - electron cloud is same on both ends: they are nonpolar