How the misuse of the Octet Rule leads to misconceptions in chemistry
This full shell or octet framework is problematic such that it results in a number of misconceptions that can hinder a student’s progress in science. In this post I will discuss how the misuse of the octet rule has lead to alternative conceptions in chemistry and chemical bonding.
Teaching the Octet Rule can be problematic
Over my many years of teaching chemistry I have noticed that many students believe that the driving force for bond formation and chemical reactions is led by the atom’s “desire” to acquire a full outer shell of electrons or noble gas configuration or an octet of electrons ( these are often used interchangeably).
Research confirms that these ideas are widespread among learners especially at the upper secondary school level (i.e. ages 14-16).
If this is something you have taught or at least alluded to in your chemistry classes you are not alone. It is an idea that is often perpetuated in many science textbooks and diagrams. Just do a quick google search and you will see the number of educational sites which promote the atom's desire to form bonds in order to acquire full outer shells or that proclaim that a chemical species is stable because it has an octet structure.
I am here to challenge some of these proclamations.
A “pedagogic learning impediment" is at play here(Taber, 2005), i.e. the way in which the subject is being taught leads to unintended and undesirable consequences for learning.
Misconceptions caused by the misuse of the octet Rule
This full shell or octet framework is problematic such that it results in a number of misconceptions that can hinder a student’s progress in science.
Taber (2002) asserts that the use of the octet rule in chemistry extends far beyond its range of application and usefulness. The misuse of the octet rule in our chemistry classes have led to (at least in part) the following alternative conceptions among our students:
Students believe that reactions occur among atoms (not molecules, ions or lattices).
Reactions occur and bonds form so that atoms can obtain full outer shells or octets
A chemical species with a full outer shell is more stable than a chemical species without one.
There are two ways that atoms can obtain full shells, by electron transfer (ionic bonding) or electron sharing (covalent bonding). This misconception will be discussed in a later post.
Why do Chemical Reactions occur? A student’s explanation.
Why do chemical reactions occur?
This simple question has quite a complex answer, and in my experience goes beyond the requirements of high school chemistry. A chemist however, would attempt to explain this in terms of bond enthalpies, entropy and ionization energy.
Students on the other hand (although they are not required to...at least in my experience) will attempt to explain chemical reactions and bond formation using the full shell principle or octet rule.
Consider this example, of a student's attempt at explaining the bonding in hydrogen fluoride:
“ Hydrogen has only one electron in its outer shell and needs two to be stable, fluorine only has seven electrons in its outer shell and needs seven. By sharing electrons both atoms will have full outer shells and will be stable because they both have noble gas configurations”
Does this sound familiar?
I am willing to bet, if you have been teaching chemistry for some time that you have encountered at least some variation of this line of thinking.
Where do these ideas come from?
It doesn’t take long to realize that many of the statements and diagrams in students’ textbooks encourage learners to think in terms of these atoms that are actively seeking to fill their shells by forming chemical bonds.
Reasons why these explanations are problematic:
1. The student assumes that the reactants in this case, hydrogen and fluorine start off as atoms when in fact, they do not. There are very few chemical processes where the reactants exist as isolated atoms (the noble gasses being the exception). Hydrogen gas and fluorine gas exist “naturally” as molecules not isolated atoms.
2. The reactants (hydrogen and fluorine molecules) and product (hydrogen fluoride), already have full valence shells, therefore, based on the student’s logic the reactants and products are already "stable".
Why then would the already stable reactants rearrange to form the product, since all chemical species involved in this reaction appear obey this full shell rule?
The octet rule or full shells explanation is not sufficient to answer this question.
Taber states that the full shell explanation is
“ inherently anthropomorphic , as no physical force is invoked to explain why systems should evolve toward certain electronic configurations. Rather, it is assumed that this is what atoms “want” and so they act accordingly…” (Taber, 2002, p. 125)
3. The idea that having an octet of electrons or a full outer shell automatically results in chemical stability is problematic as chemical stability is highly context-specific as we shall discuss.
Do full outer shells or octets present chemical stability?
Research findings show that students commonly use the octet framework/ the full shell framework to judge the stability of chemical species.
The idea that having an octet of electrons or a full outer shell automatically results in chemical stability is a persistent and widespread alternative framework.
After consulting the research and from my own experience I have summarized students' possible thought processes as follows.
Reasons why this explanation is problematic:
1. By assuming that a chemical species stability is related to its electronic configuration, students tend to ignore other important factors such as net charge, the chemical environment of the species involved and ionization energies.
2. Chemical stability is a thermodynamic concept which relates to the tendency of the state of a chemical system to change spontaneously.
In a study conducted by Taber in 2000 then repeated in 2009, researchers found that, due to their focus on the octet rule or full shell explanation, students would assign stability to questionable chemical species simply because they had octet structures.
When probed, students determined that :
‘the [sodium] atom would become stable if it either lost one electron or gained seven electrons’ (Taber, 2000, p. 485).
As seen in diagrams A and C, both of these ions have octets of electrons.
Students justified their answer by stating that the Na+ and the Na7- ions were more stable than the Na atom because these ions had full shells (or octets) unlike the Na atom.
If you would like to test your students on this, you can download The octet rule and bonding diagnostic probe for Free from my TPT store.
Reasons why this explanation is problematic:
1. Metals form cations and thus Na 7- anion is not viable
2. The Na 7- ion in fact does not have a full shell as the third shell can accommodate 18 electrons (3s2 3p6 3d10), it does satisfy the criterion of an octet however, and students determined this was a sufficient measure of stability.
3. Na 7- is chemically ridiculous and would in fact be highly unstable due to the imbalance between repulsive and attractive forces.
4. The chemical stability of the remaining two viable chemical species , the Na atom and the Na+ ion is relative to chemical context and NOT electronic configuration.
Chemical Stability is relative
Sodium metal is a highly reactive metal which must be stored under oil to prevent it from reacting with the moisture in the air. The sodium ion commonly found in chemical species such as salts and aqueous solutions is relatively stable compared to sodium metal (although it can be argued that sodium metal is made up of sodium ions). It is within this context that we can say that the sodium ion is more stable than the sodium atom. This has very little to do with the electronic configuration of either entity.
However, the isolated sodium atom is in fact more stable than the isolated sodium ion.
When measuring ionization energies, chemists often require large amounts of energy to strip an electron from the atom (because the atom is more stable than the ion).
The removed electron will however be spontaneously attracted to the sodium ion to form the atom (due to electrostatics) if no other chemical species are around. In this context, the sodium atom is more stable than the sodium ion.
When probed students have stated that the isolated sodium atom will spontaneously emit an electron from its outer shell to form the sodium ion which is more stable due to having a full outer shell, completely ignoring the concept of ionization energies preferring to use the full shell concept as an explanation.
When is the octet rule useful?
According to Taber the octet rule is a heuristic “rule-of-thumb” and states:
“the octet rule is very useful in highlighting which species are likely to be stable when comparing like with like” (Taber, 2012, p.98).
The octet rules therefore can be used to :
1. Help students predict which simple ions will likely be commonly be found
With calcium Ca (2, 8, 8, 2) the common ion would be Ca++ rather than Ca+
2. Help students determine the likely formula of simple compounds
For example the hydride of nitrogen will have the formula NH3 rather than NH2.
But there are limitations to this use as there are a number of examples of thermodynamically stable compounds that do not fit this rule such as , Aluminum trichloride and borane. Also, the transition metals commonly form a wide range of “stable” ions with different charges.
Tips for Teachers
It is without a doubt that chemical bonding is an exceptionally difficult topic to teach due to its abstract and theoretical nature. To make matters worse, teachers are expected to find ways to teach this very abstract and complex subject without many of the conceptual tools necessary to paint convincing pictures of the science, and to simultaneously make it digestible for young learners.
If like my younger self you are asking "what now?" or " how do I fix this?" I'm afraid to tell you that there isn’t a simple answer.
Researchers Joki and Aksela (2018) found that students tend to prefer an explanation where the full shell becomes the cause for what happens during chemical reactions.
The best advice that I can give based on my experience and some of the research is as follows:
1. Teach chemical bonding in terms of electrical interactions and move away from the definitions of electron transfer and electron sharing which forces students to think in terms of the full shell as an explanation for bonding. More on this in my next post when I speak about issues with how ionic bonding is taught.
2. Explain the usefulness and the limitations of the octet rule (as described) be sure to give students examples of all the exceptions to the rule and make it clear that the octet rule or the noble gas configuration or full shell principle should never be used as an explanation for chemical reactions, or chemical stability.
3. Emphasize that chemical stability is context-specific and relates to thermodynamics and not electronic configuration.
4. Although certain concepts may seem above the level of your current students’ capabilities, it is important to abstain from using language that would encourage students to think in the terms discussed in this post.
I will be exploring and discussing a number of different misconceptions in core chemistry topics in future posts. If this is something that might interest you, be sure to sign up for my mailing list for updates.
Check out the next Post in this series: Three Reasons Why You Shouldn't Teach Ionic Bonding as Electron Transfer
The Octet Rule and Bonding
Assess your students misconceptions as they relate to the octet rule and chemical bonding with this diagnostic probe. Teacher's guide included.
Joki, J., & Aksela, M. (2018). The challenges of learning and teaching chemical bonding at different school levels using electrostatic interactions instead of the octet rule as a teaching model. Chemistry Education Research and Practice, 19(3), 923. https://pubs.rsc.org/en/content/articlelanding/2018/RP/C8RP00110C#cit7
Taber, K. (Ed.). (2012). Teaching Secondary Chemistry. Hodder Education.
Taber, K. S. (1998). An Alternative Conceptual framework from Chemistry education. International Journal of Science Education, 20(5), 597-608.
Taber, K. S. (2002). Chemical Misconceptions--Prevention diagnosis and cure. Vol.1: Theoretical Back Ground. London: Royal Society of Chemistry.
Taber, K. S. (2005). Learning Quanta: barriers to stimulating transitions in student understanding of orbital ideas. Science Education, 42, 125-184.
Taber, K. S. (2009). College students conceptions of chemical stability: the wide spread adoption of a heuristic rule out of context and beyond its range of application. International Journal of Science Education, 31, 1333-1358.