Caution advised over octet rule
Bonding is an essential concept in chemistry that also presents students with many challenges. A great deal of chemistry education research has been devoted to teaching the topic and the order in which different types of bonding should be introduced. In this article, Dhindsa and Treagust make a compelling case for starting with covalent bonding followed by polar covalent and then ionic bonding.
Curriculum developers aim to sequence content so that material of increasing difficulty is covered over time, with higher level content supported by prior learning. The chosen sequence often matches the curriculum developer’s experience (or the textbook), and not necessarily that which best supports meaningful learning. In many cases, content is taught from the unknown to the known (eg ores, extraction, properties and uses), which can encourage rote learning.
Ionic bonding is typically covered first in most cases, after which covalent bonding is normally introduced almost as a distinct subject, meaning that students don’t see any association between them. This means that many students fail to appreciate what lies between these extremes of the bonding spectrum. The authors cite evidence that teachers and textbooks spend little or no time showing students that polar covalent bonding provides a link that allows students to appreciate the role of electronegativity in determining the type of bonding that exists between atoms of different elements.
An approach for teaching bonding, including accompanying analogies and explanations, is outlined in the paper. Benefits of the proposed sequence are explained clearly, illustrating that meaningful learning is promoted by progressing through concepts that build on those that precede them. As in many previous studies, using the ‘octet rule’ to teach bonding is cautioned against, since the numerous exceptions can exacerbate student misconceptions and it tends to encourage over-simplified explanations. The authors also go on to suggest how other related concepts should be introduced, included bonding in lattices and intermolecular forces.
The authors acknowledge previous work warning of potential downsides to this approach, notably the potential for students to see ionic lattices and all bonded materials as being made up of molecules, although any teacher will verify that this a common misconception regardless of the order of teaching. The approach clearly needs to be evaluated thoroughly, but the article provides a valuable introduction and sets the scene for future work.
H S Dhindsa and D F Treagust, Chem. Ed. Res. Pract., 2014 DOI: 10.1039/c4rp00059e