Polarity in Covalent Bonds

1
Polarity in Covalent Bonds

• Bonding Movie

2
Polar Bond

• Polar has poles
• ends are different
• bonds difference in how electrons distributed
• Polar bond one end more electrons than other
  end

3
Nonpolar Bond

• Nonpolar No poles
• electron cloud on one end of bond same as other
  end

4
H2 is symmetric Both ends the same electron
cloud football-shaped HCl is asymmetric
electron cloud lopsided Cl more electrons than
H
5
Red electron rich Blue electron poor
6
How can predict if BOND is polar or nonpolar?
Compare electronegativities of two atoms in bond
7
Electronegativity

• Ability of atom to attract electrons in bond

8
Electronegativities of the Elements
9
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 (?)

10
Delta notation
11
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

12
Calculate electronegativity difference for each
of these bonds
13
Bond Polarity

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

If both atoms are same element
electronegativity difference 0
14
Electronegativity Difference
15
Electronegativity Bond Type
16
Review

• Which of following bonds is most polar?
• A) O2
• B) HCl
• C) NH in NH3
• D) HBr

3.4 3.4 0
3.0 2.1 0.9
3.0 2.1 0.9
Answer (B) HCl
17
Review

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

18
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
  gt0.5 to 1.7

19
Predict the Polarity
Nonpolar

• N2
• HF
• HCl
• O2
• Cl2
• HI
• HBr

Nonpolar
20
MOLECULE Polarity

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

21
MOLECULE Polarity (3 elements)

• Depends on 2 factors
• 1. Type bonds in molecule
• 2. Arrangement of bonds or shape of molecule
• Think SYMMETRY!!

22

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

23
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!

24
Symmetric Molecules

• Contain at least two mirror planes

25
Symmetric vs. Asymmetric
CO2 is nonpolar - electron cloud is
symmetric H2O is polar - electron cloud is
lop-sided (asymmetrical)
26
Polarity of CO2?
Draw arrow along each bond pointing to more
electronegative atom If arrows cancel out,
molecule is NONPOLAR
27
Polarity of H2O?
green arrows do NOT cancel out - water is polar!
28
structural formula can be used to predict
Molecular Polarity!
H H ? C ? H H
?
?
29
Symmetry of Larger Molecules
CF4 is symmetric so is nonpolar
30
Ethane C2H6 Ethene C2H4 Ethyne C2H2
These molecules are symmetric - electron cloud is
same on both ends they are nonpolar