The full outer shells explanatory principle - Science-Education-Research

Page contents

A common alternative conception: chemical processes are driven by the desirability of full outer shells or octets

A topic in Learners' conceptions and thinking

A common alternative conception in chemistry is that processes occur because atoms need / want / desire / arrive for certain electronic configurations – in particular, octets of electrons and/or full outer shells. This conception is anthropomorphic – it treats atoms as active agents that have and act on desires.

Read about 'Widely shared alternative conceptions'

Read about 'Learners' anthropomorphic thinking'

This forms the core conception in a common alternative conceptual framework – the octet framework or octet rule framework (Taber, 1998).

Read about 'The octet rule conceptual framework'

Some examples of students’ thinking about the importance of full shells/octets

The project that was the basis for my doctoral research explored college students' (i.e., A level students in the UK) developing understanding of chemical bonding.

Read the thesis text: Understanding Chemical Bonding

"I think this atom would want to lose an electron and become an ion"
"…they [carbon and nitrogen atoms] want to fill up, like electrons on each one [orbital], to become like, stable' whereas neon has already got 'what it needs'…"
"the first shell, it needs two electrons to become stable… it joins with another hydrogen, and it shares, the other hydrogen's electron, so it thinks that it's got two electrons";

Quoted in Taber & Watts, 1996

The following links lead to some postings about student comments related to octets and full outer shells:

Some examples form other researchers

"…there is still the point that chlorine has one electron missing from a third electron shell, but potassium has a full third electron shell and one electron in the fourth electron shell, which chlorine does not…then chlorine wants to fill up its third electron shell, but potassium already has a full third electron shell, so potassium wants to donate one electron from the fourth electron shell. …Because they try to get the octet, so that they will have a certain number of electrons. "
15-16 year old Finnish upper secondary student quoted by Joki & Aksela, 2018 (translated from the Finnish by the researchers]

"They try to get to a certain energy level and they reach it by donating or receiving. …They try to reach a certain energy level…. In chemistry it is usually the octet, so that they have eight electrons in the outer shell. It is just a balanced situation which they seem to want to reach.
17-18 year old Finnish upper secondary student quoted by Joki & Aksela, 2018 (translated from the Finnish by the researchers]

Misconception in publications:

chemistry is all about elements trying to fill the outer shells of electrons / element is eagerly trying to complete its outer shell ('Superheavy: Making and breaking the periodic table')

Surely species with full shells are stable?

One heuristic of great use in chemistry is that pattern that stable species often demonstrate particular electronic configurations, and that species with full shells and/or octets of electons are often relatvie stable (i.e., the 'octet rule').

The octet rule can be very useful.

The octet rule is a useful heuristic

It can remind a student, or help them work out, that the common ion is Ca²⁺ not Ca⁺ or Ca³⁺ or Ca^– (and similarly with other simple ions).

It can remind a student, or help them work out, that the common hydride of nitrogen will be NH₃ and not NH₂ or NH₄ (and similarly with other stoichiometric relations).

It can remind a student, or help them work out, that the elements in group 18 of the periodic table (the noble gases) occur as monatomic molecules ('molecules' of a single atom), whereas other elements are not usually found in atomic form.

The octet rule, when seen in this way as a useful guide to which ions and molecules are likely to be stable, is not an alternative conception, but a useful 'rule of thumb'.

So, what is the problem?

Learners very commonly over-generalise the octet rule (assuming all species with octets/full outer shells are stable e.g., Na^7-; any species without an octet/full outer shell is not);
Learners apply the rule outside of chemical contexts (such as lattices, solvation), so expect free ions with full outer shells or octets to be more stable than the neutral atoms;
Learners see the stability of these configurations as somehow the driving force for chemical processes. Many learners assume that chemical reactions occur with reactants which are atomic (whereas they actually nearly always already comprise species that 'obey' the octet rule), and that the reactions occur to allow the atoms to obtain full-outer shells;
Often learners see anthropomorphic explanations as sufficient to explain the chemistry – the reaction occurred, the bond formed, because the atoms wanted to get full shells.

"Lithium is crucial for batteries because it is a highly reactive metal, it has an extra electron, and therefore it is happy to give that electron away, take it back again."
Jeremy Wrathall, Founder and CEO of Cornish Lithium, talking on a BBC radio programe, Lithiifed

Read about conceptions of the octet rule

The basis of an alternative conceptual framework