The Understanding Chemical Bonding project (an ECLIPSE project) explored English 'A level' (i.e. typically 16-19 year old) college students' developing understanding of the concept of chemical bonding.
The participants were interviewed at different stages of their college chemistry course about chemical bonding, and related topics.
Findings
The project suggested that students commonly entered A level study with an existing conceptual framework for making sense of chemical change and chemical bonds (the octet rule framework, based upon the core idea that bonds form and reactions occur to allow atoms to obtain full shells of electrons (the full shells explanatory principle). The findings of the project have been reported in a range of publications.
The full shells explanatory principle
The full shells explanatory principle is a common alternative conception found among learners of chemistry. The full shells explanatory principle is the core feature of the octet rule framework, an alternative conceptual framework describing common features of learners' thinking about chemistry at the submicroscopic level.
Students recognise that certain electronic arrangements had an inherent stability (often this is a reasonable interpretation of teaching), and think that a drive to acquire these arrangements as a sufficient basis for explaining chemical processes. In the Understanding Chemical Bonding project it was found that students entering a college chemistry course predominantly explained chemical bonding as being how atoms managed to fill their shells (or obtain octets of electrons or noble gas electronic configurations).
Although students might use different terminology, they generally see the same species as stable: so generally (besides period 1, hydrogen and helium) references to full shells meant atoms with an octet of outer electrons. Commonly students identify octets of electrons as full shells in the higher periods, although this is strictly only true in the second period. So the third (n=3) shell only becomes full when it has 18 electrons (3s2 3p6 3d10), but students treat the 2.8.8 configuration (Ar, Cl–, K+, Ca++ etc) as having a full outer shell.
For students adopting the full shells explanatory principle, covalent bonding forms so that atoms can share electrons to give them full shells, and the ionic bonding involves the transfer of an electron from a metal atom to a non-metal atom, to form ions with full outer shells.
(Read more about 'The full outer shells explanatory principle')
The octet rule framework
The octet rule framework or octet framework is an alternative conceptual framework, that is way of making sense of a topic that is inconsistent with scientific ideas. As the features of the framework are logically related, it is common for the same student to share several, or indeed, most aspect of the framework.
The core of the octet rule framework is the full shells explanatory principle, that suggests that chemical processes and phenomena may be explained in terms of atoms having a drive to achieve full outer shells.
Other features of the framework include:
- considering the notion of 'sharing' electrons to be a sufficient explanation of covalent bonding
- the use of anthropomorphism in discussing bonding and chemical reactions: atoms need, want, etc.;
- adopting a 'molecular framework' for ionic bonding, where ions-pairs (or other small units) are considered to be like molecules within the ionic lattice: ions are understood to form bonds by electron transfer, and ionic lattices are considered to be held together by ionic bonds (within the molecule-like units) supplemented by 'just forces' (between the molecule-like units);
- considering metallic bonding as being a type of, or hybrid of covalent/ionic bonding (i.e., electron sharing and/or electron transfer), or not to be real bonding, but just forces;
- polar bonding considered to be a variation on covalent bonding;
- hydrogen bonding misidentified as a covalent bond to a hydrogen atom;
- intermolecular bonds seen as not real bonds, but just forces;
- considering electrons to belong to atoms (and in effect to be lent out in bonding interactions);
- considering species with octets of full outer shells to be stable even if highly charged;
- considering that atoms will spontaneously ionise to obtain full shells/octets;
- considering chemical reactions to occur so that atoms can obtain full shells/octets
Student thinking is diverse, and sometimes idiosyncratic, so different students may develop different versions of the framework – including different combinations of conceptions sometimes found.
(Read more about 'The octet framework')
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