Chemistry Notes
Polarity of Molecules
Polarity
What creates polarity in a molecule?
• Migration of bond electrons
– Recall we determine the direction of this
movement with electronegativity
• Bond e- move toward the atom with a higher
electonegativity
Dipole Moments: a quantitative measure
of polarity of a bond
Polarity
Consider the Lewis structure for hydrofluoric acid: HF
• Fluorine is more electronegative than hydrogen
• Electrons shift from H to F
• The shift is symbolized by a cross arrow (
)
• The consequent charge is represented as
– δ+ - Represents a partial positive charge
– δ- - Represents a partial negative charge
Polarity
• Molecules that have partial charges are
referred to as polar molecules.
– Ionic compounds are also polar molecules
• Molecules that do not have dipole
moments are called non-polar
molecules.
– Remember dipole moments create the partial
charge
Polar Molecules
Nonpolar Molecule
Polarity
Determining Polarity
• If all bonds are nonpolar, the molecule is
non-polar
• If at least one bond is
polar, the molecule is
polar, UNLESS it is
symmetrical, then it is
non-polar
Consider the molecule NH3
• N is more electro-negative
than H
• Electrons move closer to N
• Unshared electrons on N
also have a dipole moment
(up)
• NH3 has an overall polarity
upwards
Consider the molecule NF3
• N is less electronegative than F
• Electrons move
closer to F
• Unshared electrons
on N also have a
dipole moment (up)
• NF3 has an overall
polarity downwards
Polar v. Nonpolar
Polar v. Nonpolar
Summary
Be able to…
• Explain what creates polarity
• Determine a dipole
– Using the cross-arrow
– Assign partial charges
• Tell whether a molecule is polar or
nonpolar