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.
The full shells explanatory principle was identified during the Understanding Chemical Bonding project.
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.
Some examples from the ECLIPSE projects data:
A sodium atom wants to donate its electron to another atom
The chlorine atoms share electrons to fill in their shells
The electron on the outer shell of an atom of sodium will fall out
(The findings discussed here have been reported in the research literature. Details of Dr Taber's publications may be found here)
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Dr Keith S Taber kst24@cam.ac.uk
University of Cambridge Faculty of Education
© Keith S Taber, 2012-2013