Covalent Bonding
Molecular
Polarity
I
II
III
Covalent Bonds involve sharing electrons
But
Just like in real life, not all sharing
Is equal !!!
/
Electronegativity
Electronegativity is the ability of an atom to
Attract electrons to itself in a covalent bond
Large Electronegavity
Small Electronegativity
So, the atoms in a covalent bond are
engaged in a tug-of-war over electrons
electrons
Results of Tug of War
The atom that “wins”
will be the one that is
more electronegative.
Because this atom
gets the electrons
more of the time, it
becomes slightly
negative
The atom that “loses”
becomes slightly
positive
The Periodic Table allows us
To predict differences in
F is the
most
electronegavity
electronegative
A. Dipole Moment

Means the direction of the polar bond
in a molecule.

Arrow points toward the more
electronegative atom. (The direction
the electrons will flow
+

H
Cl

B. Determining Molecular Polarity

Depends on:
 Electronegativity differences
 dipole moments
 molecular shape
B. Determining Molecular Polarity

Polar Molecules
 Dipole moments are asymmetrical
and don’t cancel .
O
H2O
H
H
net
dipole
moment
B. Determining Molecular Polarity

Nonpolar Molecules
 Dipole moments are symmetrical
and cancel out.
F
BF3
B
F
F
B. Determining Molecular Polarity

Therefore, polar molecules have...
 asymmetrical shape (lone pairs) or
 asymmetrical atoms
H
CHCl3
Cl
Cl
Cl
net
dipole
moment
Linear molecules can be nonpolar . . .
Example: CO2
What makes
the difference?
Or Nonpolar.
No Net Dipole Moment
Example: HCN
Net Dipole Moment Towards N
Trigonal Planar molecules are usually nonpolar
Example: BF3
But, what makes this one polar?
Example: BF2Cl
Trigonal Pyramidal Molecules are Polar.
Example: NH3
Net Dipole
Toward N
Tetrahedral Molecules can be polar
Example: CH3F
Or non polar
Example: CF4
Explain this!
Can a Molecule be Both Polar
and NonPolar?
Trigonal planar with
Asymmetric atoms
Tetrahedral with
symmetric atoms
Non Polar End
Polar End
So, polarity can be more of a
spectrum
than a black and white
determination
Polar------------------------------------------------------nonpolar
More Nonpolar Molecules
•Share electrons the “best”
•Have the closest electronegativities (or smallest differences)
•Have lower boiling and melting points
•. . .so More likely to be gases at room temperature
•Like dissolves like . . .
• Use a nonpolar solvent to dissolve a nonpolar solute
More Polar Molecules
•Do not share well at all!
•Their electronegativity differences are great
•Are more like ionic compounds
•Have higher melting and boiling points
•Most likely to be liquids at room temperature
•Like dissolves like . . .
•Use a polar/ionic solvent to dissolve a polar/ionic solute
Ionic, Polar, Nonpolar: A Continuum
Ionic------------ ------------Polar---------
-------Nonpolar
Electrons transferred E- shared unequally
Huge charge separation
E- shared
no charge sep.
some charge sep.
High mp/bp
Solids at STP
Low mp/bp
Liquids at STP
Gas at STP