I II III

Molecular Polarity

Covalent Bonding

Covalent Bonds involve sharing electronsButJust like in real life, not all sharingIs 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 usTo predict differences inelectronegavity

F is the most 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 .

netdipolemoment

H2OH H

O

B. Determining Molecular Polarity Nonpolar Molecules

Dipole moments are symmetrical and cancel out.

BF3

F

F F

B

CHCl3

H

Cl ClCl

B. Determining Molecular Polarity Therefore, polar molecules have...

asymmetrical shape (lone pairs) or asymmetrical atoms

netdipolemoment

Example: CO2

Example: HCN

Linear molecules can be nonpolar . . .

Or Nonpolar.

What makes the difference?

No Net Dipole Moment

Net Dipole Moment Towards N

Trigonal Planar molecules are usually nonpolar

But, what makes this one polar?

Example: BF3

Example: BF2Cl

Trigonal Pyramidal Molecules are Polar.

Example: NH3

Net DipoleToward N

Tetrahedral Molecules can be polar

Or non polar

Example: CH3F

Example: CF4

Explain this!

Can a Molecule be Both Polar and NonPolar?

Non Polar EndPolar End

Tetrahedral with symmetric atoms

Trigonal planar withAsymmetric atoms

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 E- shared

Huge charge separation no charge sep.

some charge sep.

High mp/bp Low mp/bp

Solids at STP Liquids at STP Gas at STP