A topic in Learners' conceptions and thinking
Two types of chemical bond
In elementary chemistry learners often acquire there idea that there are two types of chemical bond: covalent and ionic.
A teacher might think this is a harmless simplification or even a basic model that can be developed later. However, this idea seems to often have a strong intuitive hold over some learners so they struggle to move beyond it. So, if this idea is taught as an introductory teaching model it can actually act as a pedagogic learning impediment that interferes with later learning.
A study with A level students studying chemistry found that:
"Students tended to class bonds as either covalent or ionic, even when they had been taught about electronegativity and polar bonding. At the beginning of the A- level course students commonly only recognized these two categories, which could clearly be explained in terms of the full shells explanatory principle. Even when the interviews showed that students could discuss bond polarity, they spontaneously tended to class polar bonds as covalent (e.g. Al–Cl, B–F, Ca–Cl, C–Cl, C-O, H-F, Si-O, Ag-Cl) or ionic (e.g. Al-Cl, B-F, C-Cl, H-Cl, H-F). Sometimes students suggested that metals also had covalent or ionic bonding.
When students learned about dative bonds they tended to consider these as 'a type of covalent bonding' or 'a form of covalent bonding', rather than a polar bond. Although one atom donated both electrons it was considered that 'once a dative bond is formed it's the same as a covalent bond'."
Taber, 1998, p.605
Why is this simplification not easily given up?
In teaching we often use simplifications and basic models as stepping stones towards more more nuanced and sophisticated teaching. Often this works well, But some simplification and models used in teaching seem to become readily adopted by learners and difficult to move on form.
Possibly there is something about dichotomies that make them easy to understand (good versus evil; metals versus non-metals)?
However, learners often have alternative conceptions of covalent and ionic bonding that make good sense to them, and where intermediate and other classes do not seem to fit:
- covalent bonding – is often understood as two atoms sharing electrons to obtain an octet or full outer shell
- ionic bonding is often understood as produced by an electron transfer from one atom to another, so both atoms can obtain octets or full outer shells
(It is not so clear to learners how polar bonds or hydrogen bonds, for example, can be explained in terms of forming octets.)
Read about the alternative conception that ionic bonding depends on electron transfer between atoms
What are the consequences?
All learners are different, and they vary in how much they commit to ideas and how easily they accept alternatives. Common issues are:
Metallic bonding
Students may think that
- there are no chemical bonds in metals as metals are elements and so do not need bonds as they are atomic;
- there are forces/interactions in metals, but these do not count as proper chemical bonds;
- the bonding in metals is covalent / a form of covalent bonding;
- the bonding in metals is ionic / a form of ionic bonding;
- the bonding in meals is a mixture or hybrid of covalent and ionic bonding
Learners may also simply explain there is bonding due to the 'sea of electrons' but sometimes this is little more than a rote learned slogan without nay further appreciate of what this 'sea' is and 'why' it might lead to bonding (Taber, 2003)
Polar bonds
From the canonical view, purely covalent and purely ionic bonds are kinds of ideals (there are pure covalent bonds in elements, but there are no examples of pure ionic bonds in nature), and most bonds have both some ionic and some covalent character and so it is a matter of degree – a point on a continuum. (Indeed many bonds are considered to also have substantial metallic, ionic and covalent character, and can be located in a 'bonding triangle' to show the relative strengths of each character.)
Chemical bonding is best understood in terms of the forces acting between the different charged components of molecules or lattices.
However if learners have been taught there is a bonding dichotomy (chemical bonds are ionic or covalent – see Table 2 below) and have committed to this idea, they may then find it difficult to shift to thinking of a continuum between two ideal models (see Table 3 below). One common outcome is that when taught about polar bonds, these are understood as being not between covalent and ionic but rather as a special type of covalent bond or ionic bond (e.g., see Table 4 below).
Ionic bonds
If learners adopt a bonding dichotomy where ionic bonds are due to electron transfer, they may think that the ionic lattice contains a mixture of ionic bonds and just forces: e.g., in NaCl each Na+ ions is bonded to the Cl– ion it donated an electron to and just attracted by forces to other anions.
Hydrogen bonds
When learns are committed to the bonding dichotomy
- if hydrogen bonds are not understood to be formed by electron transfer or electron sharing, they may be seen as not a proper chemical bond
- hydrogen bonds may be assumed to be just bonds to a hydrogen atomic centre, and therefore just a type of covalent bond
Work cited:
- Taber, K. S. (1998). An alternative conceptual framework from chemistry education. International Journal of Science Education, 20(5), 597-608. [Download paper]
- Taber, K. S. (2003) Mediating mental models of metals: acknowledging the priority of the learner's prior learning, Science Education, 87, pp.732-758. DOI 10.1002/sce.10079 [Download paper]
- Taber, K. S. (2008) Of Models, Mermaids and Methods: The Role of Analytical Pluralism in Understanding Student Learning in Science, in Ingrid V. Eriksson (Ed.) Science Education in the 21st Century, pp.69-106. Hauppauge, NY: Nova Science Publishers. [Download chapter]
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