Evidence of chemical bonding facts sense-making (CBF)

Chemical bonding facts sense-making (CBF) involves students making talk and visuals about abstract chemical bonding concepts of chemical processes and objects. This implies that good understanding of abstract scientific words is a sign that sense-making of chemical knowledge is advancing. The frequent use of abstract scientific concepts in this type of sense-making makes it more aligned to scientific facts and rules than the perceptual sense-making (Zimmerman et al, 2009). Therefore, I analysed this sense-making type in this study in order to understand how learners cope with abstract scientific concepts and processes before and after employing the intersemiotic complementarity teaching approach. Overall, I observed that learners have little chemical bonding knowledge at this sense-making level – evident in them being unable to use abstract concepts and ideas adequately when explaining chemical phenomena.

90 I found that more than half of the class (more than 19 learners) had difficulty explaining the microscopic conditions that lead to chemical bonding. Learner W incorrectly explained that

“elements that are bonding is because they are in the periodic table to react”. When asked to state, with a reason, whether argon and fluorine can bond, Learner A incorrectly answered that “yes, because they all non-metals that share electrons and can form a covalent bond”.

These two excerpts of learners’ talk reveal that learners lack the knowledge of chemical bonding occurring due to incomplete (unstable) outer shells of atoms of some elements. The learner expressing that argon and fluorine bond covalently because they are non-metals shows his mere understanding of this bonding type happening between non-metals. Though he could use the concept ‘sharing’, he did not think of argon being unreactive due to the full outer shells that its atoms have. This reveals that his knowledge of more abstract concepts and ideas (such as stable/complete outer shells; inert/noble gas) is limited.

Learner P defined a diatomic molecule as “elements that found with +2”. After a series of follow-up questions posed by the teacher, two possible reasons emerged for his failure to answer the question correctly. First, it is possible that this learner did not know either the meaning of the prefix di- or the meaning of the concept molecule, both of which are more abstract than perceptual. Second, this learner seemed to have interchangeably used knowledge of diatomic molecules with charges on ions. He could understand that the prefix di refers to two things, but ended up wrongly linking it to the charges formed during ionic bonding. Learner A correctly explained that a sodium atom loses one electron during its bond with a chlorine atom. However, he wrongly referred to an ion formed by a sodium atom as an anion, when he should have referred to it as a cation. I see this knowledge as constrained possibly by the verbal mode used and by it being sub-microscopic, as electrons are invisible to the naked eye.

The learners’ inability to make sense of chemical bonding facts was also identified in their visual representation of chemical diagrams of a covalent bond in carbon dioxide and chlorine molecules. Though it was evident that some learners generally know that covalent bonding involves electrons sharing, specific difficulties could clearly be identified from the Bohr diagrams of carbon dioxide and nitrogen molecules they drew. For example, some of these difficulties could be identified from Learner F’s incorrect bond diagram of a carbon dioxide molecule in Figure 5.

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Figure 5. An incorrect bond diagram of a carbon dioxide molecule (drawn by Learner F after Cycle 1)

First, this learner shows lack of understanding of covalent bonding in carbon dioxide in the incorrect number of atoms he has drawn. The molecule of carbon dioxide (CO2) actually consists of three atoms in total: one carbon atom, and two oxygen atoms. However, the Bohr diagram of a carbon dioxide molecule (Figure 5) drawn by this learner has four atoms instead of three. Second, the atoms in the molecule are not labelled, and the number of electrons shared is shown incorrectly. They could have labelled the atoms involved in the bond, as this would indicate their knowledge of atoms contained in a carbon dioxide molecule. This is good, because in Namibian schools, marks are also awarded for correct labelling of atoms in a molecule. Carbon atoms share four electrons because their valency is four, while oxygen atoms share two electrons because their valency is two. This knowledge is abstract, as learners do not observe a carbon dioxide molecule and so can access this knowledge only via the teacher’s explanation. If the teacher used a physical model of a carbon dioxide molecule, this learner could have possibly constructed a more meaningful Bohr structure of a carbon dioxide molecule than the one in Figure 5.

Learner X’s Bohr diagram of a nitrogen molecule (Figure 6) shows his more correct mental model of covalent bonding. However, the details of the bond diagram he drew show that his understanding of covalent bonding was still limited.

92 Figure 6. An incorrect bond diagram of a nitrogen molecule (drawn by Learner X after Cycle 1)

This learner showed that he understood that nitrogen is a diatomic molecule. This was shown in the two nitrogen atoms bonded together that he drew. He also knew that both atoms of nitrogen are non-metal and share electrons. His diagrams are correctly labelled, indicating his awareness of the need to show to the viewer/reader all atoms that make up a molecule.

However, he lacks knowledge of the valency concept, and how it is applied to determine the number of electrons shared by two atoms that bond.