ChemistryChemistry

Molecular PolarityMolecular Polarity

Just like bonds can be polar because of Just like bonds can be polar because of un-equal electron distribution, un-equal electron distribution, molecules molecules can be polarcan be polar because of un-equal because of un-equal electron cloud distributionselectron cloud distributions

These imbalances are not the same as ion These imbalances are not the same as ion formation.formation.

How do we know when a molecule is How do we know when a molecule is polar?polar?

Molecular PolarityMolecular PolarityPolarity in a molecules determines Polarity in a molecules determines whether or not electrons in that whether or not electrons in that molecule are distributed equally.molecule are distributed equally.

When determining the polarity of a When determining the polarity of a molecule, it is all about symmetry. molecule, it is all about symmetry.

AsymmetricAsymmetric molecules tend to be molecules tend to be polarpolar..

Symmetric Symmetric molecules are always molecules are always non-non-polar. polar.

When determining the polarity of a When determining the polarity of a molecule, follow these steps:molecule, follow these steps:

Draw the Electron Dot structure of the Draw the Electron Dot structure of the molecule.molecule.

Using the electronegativity chart determine Using the electronegativity chart determine the difference in electronegativity for each the difference in electronegativity for each bond.bond.

0—0.4 = Non-polar0—0.4 = Non-polar 0.5—1.7 = Polar0.5—1.7 = Polar

The molecule is non-polar or polar based The molecule is non-polar or polar based on the following criteria:on the following criteria:

The molecule is The molecule is non-polarnon-polar if either: if either: each bond in the molecule is non-polar and each bond in the molecule is non-polar and

there are there are no un-bonded electron pairs on no un-bonded electron pairs on the center atomthe center atom. (example: CBr. (example: CBr44))

Each C-Br bond is non-polar (Each C-Br bond is non-polar (ΔΔEN=0.3), no side of EN=0.3), no side of the molecule has more electron density. It is the molecule has more electron density. It is symmetrical!symmetrical!

each bond in the molecule has the each bond in the molecule has the samesame polaritypolarity and there are and there are nono un-bonded un-bonded electron pairs on the central atom. electron pairs on the central atom. (Example: CF(Example: CF44))

C-F bond is polar C-F bond is polar ΔΔEN=1.5,EN=1.5, But, Molecule is symmetrical But, Molecule is symmetrical

The molecule is The molecule is polarpolar if: if:each bond in the molecule is each bond in the molecule is non-polar, but there non-polar, but there are un-are un-bonded electron pairs on the bonded electron pairs on the central atom. central atom.

Example: NBrExample: NBr33

bonds in the molecule have bonds in the molecule have different polarities different polarities and/or and/or there there areare un-bonded electron un-bonded electron pairs on the central atom. pairs on the central atom. Example: CHExample: CH33BrBr

Molecular Polarity SummaryMolecular Polarity Summary

Molecule Polarity

The H─Cl bond is polar. The bonding electrons are pulled toward the Cl end of the molecule. The net result is a polar molecule.

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Molecule Polarity

The H─O bond is polar. Both sets of bonding electrons are pulled toward the O end of the molecule. The net result is a polar molecule.

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Molecule Polarity

The O─C bond is polar. The bonding electrons are pulled equally toward both O ends of the molecule. The net result is a nonpolar molecule.

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S C O

COS is linear. C and S have the same EN (2.0) but the sulfur atom is bigger than the oxygen side (more electron density),so the molecule is polar overall.

Polar or Non-Polar?Polar or Non-Polar?SOSO22

PFPF33

BClBCl33

BHFBHF22

Polar (Polar (unbonded e- on center unbonded e- on center atom!)atom!)

Polar (Polar (unbonded e- on center unbonded e- on center atom!)atom!)

Non Polar Non Polar (bonds are (bonds are polar, but symmetrical)polar, but symmetrical)

Polar – different bond Polar – different bond polaritiespolarities

More Molecular Polarity…More Molecular Polarity…

Molecular 3D Models website