Polarity in Covalent Bonds
Bonding Movie
Polar Bond
• Polar – has poles
–ends are different
–bonds: difference in how electrons
distributed
–Polar bond: one end more electrons
than other end
Nonpolar Bond
• Nonpolar = No poles
• electron cloud on one end of bond same
as other end
H2 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 = H2
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
• 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
LiH : 2.1 – 1.0
1.1
H2 : 2.1 – 2.1
0
HF: 2.1 – 4
1.9
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
Electronegativity
Difference
Type of Bond
0.0 to 0.5
Nonpolar Covalent
>0.5 to 1.7
Polar Covalent
> 1.7
Ionic
Electronegativity & Bond Type
Review
• Which of following bonds is most polar?
– A) O2
3.4 – 3.4 = 0
– B) HCl 2.1 – 3.2 = -1.1 = 1.1
– C) NH in NH3 3.0 – 2.1 = 0.9
– D) HBr 3.0 – 2.1 = 0.9
Answer = (B)
HCl
Review
• Which substance contains bond with
greatest ionic character?
– A) KCl 0.8 – 3.2 = -2.4 = 2.4
– B) HCl 2.1 – 3.2 = -1.1 = 1.1
– C) Cl2 3.2 – 3.2 = 0
– D) CCl4 2.6 – 3.2 = -0.6 = 0.6
Answer = (A)
KCl
Summary
• 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 Polarity
•
•
•
•
•
•
•
N2 Nonpolar
HF Polar
HCl Polar
O2 Nonpolar
Cl2 Nonpolar
HI Polar
HBr 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
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
CO2 is nonpolar
- electron cloud is symmetric
H2O is polar
- electron cloud is 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
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