Integrated SequencePhysics Chemistry Organic Biology

Web Resources

Chem1 Virtual Textbook - Polar and nonpolar bonds
Excellent discussion of the role of electronegativity difference in determining the character of a chemical bond.

Purdue University - Electronegativity
Defines electronegativity and discusses the use of electronegativity difference to identify identify ionic, covalent, and polar covalent compounds.

HyperPhysics - Polar Covalent Bonds

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Special points of emphasis

Periodic Properties

The Chemical Bond

Intermolecular Forces

Functional Groups in Organic Chemistry

It is easy to become accustomed to thinking only about whether or not the bonds within a molecule are polar in deciding the intermolecular force relationships it can engage in. Typically, you examine the electronegativity differences between bonded atoms and assign the polarity of the bonds and go from there to decide whether the molecule is nonpolar or polar.

However, the polarity of bonded atoms is not the only factor in determining the overall polarity of a molecule, and by extension, intermolecular force. Molecular geometry as predicted by VSEPR is also important.

For example, if the various dipole moments within a molecule are oppositely directed in space, even though the particular bonds are polar, the molecule may have no net dipole moment at all, in which case the degree of intermolecular force is weak. Carbon dioxide is the quintessential example, a linear molecule in which the dipole moments of the carbon-oxygen bonds are oppositely directed.

From a test-writer's perspective, this kind of exception makes a good multiple choice question. Keep VSEPR in mind on the MCAT if you are faced with interpreting the overall polarity of a molecule. Don't forget to ask yourself whether the dipole moments of individual bonds cancel each other out.

Work, Energy, and Power


The Chemical Bond



Within the context of our discussion of covalent bonding, let us reprise an important point regarding the underlying coherence of the oxidation-reduction method of describing chemical reactions. In covalent bond formation, internal energy decreases as electrons form molecular orbitals and draw the nuclei inward along the bond axis to form the bond between the atoms. Further energy is lost by the system when a polar bond forms as the bonded electrons are drawn in toward the more electronegative atom.

Polar bond formation represents an additional internal energy decrease. For this reason, polar bonds tend to be stronger than non polar bonds. This underlying concept is the basis for oxidation-reduction. The graphs of the electronegativities of the elements and their reduction potentials closely agree. Electronegative atoms pull electrons in closely when they form bonds, which is a favorable development from the point of view of energy.

This is the true inside story of redox. Oxidation-reduction gives a systematic way to account for this phenomenon by representing the chemical reaction process as a narrative of 'electron control' and standardizing the energy involved in tables of reduction potentials.

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