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Molecular Polarity
• Polar describes a bond where one atom has a
partial positive charge and the other a partial
negative charge
• Molecules as a whole can be polar
• In a polar molecule electron density
accumulates toward one side of the molecule
• Experimental measurement of polarity of a
molecule
Molecular Polarity
• When polar molecules are placed in electric
field they experience a force that aligns them
with the field
• Electric field is created by pair of oppositely
charged plates, positive end of molecule is
attracted to negative plate and vice versa
• The extent to which the molecules line up
with the field depends on their dipole
moment µ
Molecular Polarity
defined as the product of the magnitude of the
partial charges (ᵟ+ and ᵟ-) & the distance they
are seperated
SI unit is coloumb-meter
Force of attraction b/w negative end of one
polar molecule and positive end of another is
dipole-dipole force
Intermolecular forces influence the temperature
Molecular Polarity
• At which a liquid freezes or boils
• The forces determine whether a liquid
dissolves certain gases or solids or whether it
mixes with other liquids
• To predict whether a molecule is polar need to
consider if it has polar bonds and how these
bonds are positioned
• Eg diatomic molecules composed of two
atoms with different electronegativities are
always polar
• What happens to 𝐴𝐵2 , 𝐴𝐵3 or 𝐴𝐵4
• Will evaluate how the choice of substituent or
terminal groups (B) and molecular geometry
influence molecular polarity
• 𝐴𝐵2 eg 𝐶𝑂2 the the C and O pond is polar. The
terminal atoms are at the same distance from
C and both have ᵟ- charge and is symmetrically
arranged around C atom, therefore 𝐶𝑂2 has
no molecular dipole even though each bond is
polar.
• 𝐻2 𝑂 bent triatomic molecule, O has larger
electronegativity then H each O-H bond is
polar
• Electron density accumulates on O side
making the molecule electrically lopsided and
therefore polar