Teaching Formal\nCharges of Lewis Electron Dot Structures\nby Counting Attachments

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