Teaching Formal Charges of Lewis Electron Dot Structures by Counting Attachments

A method of assigning formal charges from counting “attachments” for visual inspection is introduced as part of the iterative portion of constructing Lewis electron dot structures. An “attachment” is either a “line” for a bond or a “dot” for a single electron, which is compared against the number of valence electrons, as indicated by group number. While formal charges are assigned for formality only, they are a useful tool to rationalize the more likely Lewis structure, especially when considering relative electronegativity differences among the elements. This method describes how changing the number of attachments around an atom in the Lewis electron dot structure fosters facile pattern recognition. The “attachment method” removes the accounting of numbers in categorizing electrons of “nonbonding” vs “bonding” in the more prevalent mathematical equation and may be useful for teaching introductory general chemistry. Various examples are described in contrasting the traditional mathematical approach and this “attachment method”. The goal is to provide another means for teaching formal charges, especially to reach students with various learning preferences. In surveying 426 students in a general chemistry course, a majority prefers the attachment method, and this group reports a statistically modest gain in performance on the corresponding in-class assessment.