Polarity of Molecules: A New Approach for Freshman Chemistry

Abstract

Polarity of molecules is one of the central concepts that is used for predicting or rationalizing a wide array of concepts in the undergraduate chemistry curriculum. These concepts include the physical properties of molecules at the freshman level, structure-reactivity relationships in organic and inorganic chemistry, and selection rules in spectroscopy. In all of the freshman chemistry textbooks that we examined, bond dipoles are assigned based on electronegativity differences, which are then added vectorially (sometimes implicitly) to obtain the net dipole moment for a given molecule. We have found that this approach, which relies on the students’ familiarity and comfort level with vector addition in three dimensions, works well only for a small fraction of our students. We present an alternative pedagogical method for conveying this important concept. This method does not invoke vector addition, relying instead on a simple, pictorial approach that is transparent to a majority of our students. This method applies not only to species with a clearly defined central atom but to substituted alkenes and substituted benzenes as well. In addition, it can also be extended to paramagnetic species, whose polarity is not generally discussed in most freshman chemistry textbooks.