A topic in science concepts and learners' conceptions and thinking
Stability in chemistry
Something that is stable does not tend to change. In chemistry stability of usually associated with energetic/thermodynamic considerations.
Stability should really be considered in a specific context. A pellet of sodium stored in oil may be considered relatively stable, whereas a similar pellet left exposed to moist air would not.
Stability should also be distinguished from inertness.
Stability refers to energy states (a stable system is at a lower energy and needs an energy input to disrupt it)
Inertness refers to the kinetics of change – a labile system is one where change occurs at a high rate.
An unstable system may be inert because there is a large energy barrier to change, so although it could theoretically evolve to a lower energy system, this requires a high activation energy.
Stability and inertness refer to different aspects of systems (stability relates to thermodynamic considerations; inertness relates to kinetic considerations)
Diamonds are not forever
Diamonds form deep in the earth, under conditions where diamond has a stable structure. At the earth's surface, diamond is not a stable structure, as it can spontaneously change to a lower energy arrangement: graphite. Yet there is a high energy barrier to this rearrangement, so the rate at which diamonds change into graphite, is (on a human scale) very, very low. In terms of a human lifetime, diamonds last; but in chemical terms diamonds are not forever.
At normal atmospheric conditions diamonds are unstable, but very inert. Sometimes the diamond structure is said to be 'metastable' – technically it is not stable, but for most practical purposes it can be taken as if stable.
Alternative conceptions of chemical stability
Research to explore learners' thinking suggest they often have rather unhelpful notions of chemical stability.
Research (e.g., Taber 2009) has shown that many learners at school and college level use a simple heuristic to decide which chemical species are stable – the octet rule. This rule is indeed commonly used to suggest which chemical species might be stable, and can sometimes be very useful:
- the octet rule tells us to expect CH4 and not CH3 as the stable hydride of carbon
- the octet rule tells us to expect O2- and not O3- or O2+ to be the common ion of oxygen
However, students often apply the octet rule in contexts where it is not useful, as the examples below suggest.
Read about the octet alternative conceptual framework
Comparing the stability of a sodium atom and sodium ions
Consider the image below that was used in a diagnostic instrument prepared for use in schools.
If asked about the relative stability of these three species, a chemist or science teacher might suggest:
- The Na+ ion is a commonly found species as (although it has a charge) it is stable in contexts such as ionic or metallic latices or in solution when it is hydrated.
- However, in a direct comparison with the atom, the ion will be less stable – as we know it requires an energy input (ionisation energy) to pull the electrons away form the rest of the atom
- The Na7- ion is clearly not a stable species. As a metal, sodium forms cations; and the more highly charged nan ion, the harder it is to stabilise.
In practice, students commonly offer different views. Interviews with students had elicited comments that suggested students often had a notion of chemical stability strongly aligned with the octet rule, and which could be summarised:
octet/full outer shell = stable
a simple (and misleading) heuristic commonly applied by students
otherwise not stable
The diagnostic instruments were designed to test the generality of this alternative conception.
"This original version of the Chemical Stability Probe (Taber, 2000) was administered to two A-level chemistry groups in the same institution…: students who had already studied relevant topics (e.g. ionisation energies) at A-level standard, and students just commencing A-level studies."
Taber, 2009, p.1336
It was found that
"most students saw the cation as more stable than the atom, and–of particular note–that most also saw the atom to be less stable than the anion. It is also of interest that at least one-half of the students in each group thought that the anion was as stable as the cation
Taber, 2009, p.1336
An example of student reasoning was:
[The cation] is more stable than [the atom] because its outer shell electron has eight electrons and is full whereas [the atom] only has one electron in its outer shell and is therefore less stable.
[The atom] is less stable than [the anion] because again the outer shell of [the anion] is full with eight electrons but [the atom] only has 1 electron in its outer shell and is less stable.
[The anion] and [the cation] are equally stable because both outer shells are full and the valency requirements have been fulfilled. Therefore both are equally stable.
Advanced level chemistry student quoted in Taber, 2009, p.1338
When a version of the instrument 1 was administered to a "teaching group comprising 19 students studying college-level chemistry (i.e., A-level) at an English further education college that had not previously been involved in the research", similar findings were obtained.
"In comparing the cation and atom, it made little difference which way round the question was worded–with 8/9 (original wording) and 9/10 (revised wording) students responding that the cation was more stable. Similarly in the second comparison, 7/9 and 10/10 felt that the anion was more stable than the atom. (In the final comparison there was more of a distinction, with 4/9 and 9/10 responding that the two ions were equally stable.)"
Taber, 2009, p.1341
Again, it was common for students to explain stability in terms of octets or full shells. So for the the highly unstable Na7- anion:
- "Na7- has a noble gas configuration also (isoelectronic with Ar), therefore stable compared to Na, with 1 outer electron."
- "[Na7- anion] is more stable than [Na atom] because [Na atom] only has one electron in its outer shell, but [Na7- anion] has 8 making it stable."
- "[Na7- anion] is more stable than [Na atom] because [Na7- anion] has full outer shell."
Logically, if students think Na+ is more stable than Na, they should expect the sodium atom to spontaneously emit an electron, But, surely they would NOT think that…or would they?
What would spontaneously occur?
Consider the image below that was used in a diagnostic instrument prepared for use in schools. The image shows two systems comprising of the same particles:
- a sodium ion (Na+) a short distance from a free electron
- a sodium atom (Na)
From a canonical perspective, the positive ion and negative electron should attract and spontaneously move towards each other, whereas work would have to be done (i.e., ionisation energy) to remove an electron from the atom. So, the atom is more stable than the separated ion-electron pair.
Yet, it is very common for students to take a very different view.
"The probe was administered to three A-level teaching groups in institutions in England..two school sixth forms, and a further education college….It was found that most of the students considered the sodium ion to be more stable than the sodium atom…In Question 2, a majority of the respondents selected the option that a sodium atom would emit an electron, about three times as many as thought that an ion would combine with an electron to form an atom …This suggests that most of the students judging the ion to be more stable do interpret this in a similar sense to its formal meaning, and are not just using 'stable' as a label for a certain types of structure."
Taber, 2009, p.1344
When asked to choose between a set of statements it was found:
| Statement | Selected by |
| The sodium atom is more stable than the sodium ion | 3 |
| The sodium ion is more stable than the sodium atom | 24 |
| The sodium ion and sodium atom are equally stable | 0 |
| I do not know which statement is correct | 1 |
| Total | 28 |
| Statement | Selected by |
| The sodium atom will emit an electron to become an ion | 18 |
| The sodium atom and electron will combine to become an atom | 5 |
| Neither of the changes suggested above will occur | 3 |
| I do not know which statement is correct | 2 |
| Total | 28 |
More stability triads to compare
We might wonder if there is something special about the example of sodium that leads to students assuming the atom is less stable than the cation (or even the anion). After all, the Na+ cation is very familiar in the lab. in salts and solutions, so we might appreciate why it is associated with stability.
Several more triads were added to the diagnostic instrument reported above.
"The expanded set of probes (Taber, 2002) was administered in five further institutions (three school sixth forms, a sixth-form college and a further education college) to groups of students studying A-level chemistry. The total sample size was 152 students (with samples from within each institution varying in size from 18 to 58).
As the five different probes in the set were provided already sequenced, teachers were asked to simply distribute probes around the class from the deck of probes. This meant that each student would be answering different questions from others around them, and that approximately equal numbers of each probe would be completed."
Taber, 2009, p.1347
Here are some of the key findings:
Most students though the Na7- anion was more stable than the atom
| Judgement of relative stability | Selection of judgement | Octet thinking justification |
| Na+ is more stable than Na | 27/33 | 22/27 |
| Na7- is more stable than Na | 21/33 | 17/21 |
Most students think a beryllium cation is more stable than the neutral atom
| Judgement of relative stability | Selection of judgement | Octet thinking justification |
| Be2+ is more stable than Be | 20/27 | 13/20 |
| Be is less stable than Be6- | 13/27 | 13/13 |
Most students think both a C4+ carbocation and C4- carbanion are more stable than a carbon atom
| Judgement of relative stability | Selection of judgement | Octet thinking justification |
| C4+ is more stable than C | 17/30 | 16/17 |
| C is less stable than C4- | 17/30 | 15/17 |
Many students think a Cl11- anion will be more stable than the atom
| Judgement of relative stability | Selection of judgement | Octet thinking justification |
| Cl11- is at least as stable as Cl– | 11/31 | 8/11 |
| Cl11- is more stable than the Cl atom | 12/31 | 9/12 |
Most students think an excited electronic state with full outer shell will be more stable than the ground state!
| Judgement of relative stability | Selection of judgement | Octet thinking justification |
| [more excited state] Cl (1.8.8) is at least as stable as [less excited state] Cl (2.7.8) | 20/31 | 14/20 |
| [excited state] Cl (2.7.8) is more stable than the [ground state] Cl (2.8.7) | 14/31 | 11/14 |
| [ground state] Cl (2.8.7) is less stable than the [excited state] Cl (1.8.8) | 17/31 | 12/17 |
Replication?
Joki & Aksela report a 3rd year upper-secondary Finnish student (c.17-18 years of age) presented with the Na / Na+ / Na7- triad as saying "
"Yes, sodium 1+ is the most stable…There is just in this case the well-known octet in the outer shell, and so it is the mostable [sic] in terms of energy."
Joki & Aksela, 2018, p.937 (reported in translation from the Finnish)
Work cited
- 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 [10.1039/C8RP00110C]. Chemistry Education Research and Practice, 19(3), 932-953. https://doi.org/10.1039/C8RP00110C
- Taber, K. S. (2002) Chemical misconceptions – prevention, diagnosis and cure: Volume 2: classroom resources, London: Royal Society of Chemistry .
- Taber, K. S. (2009) College students' conceptions of chemical stability: The widespread adoption of a heuristic rule out of context and beyond its range of application. International Journal of Science Education, 31(10), 1333-1358. doi: 10.1080/09500690801975594. (Download the paper)
Note:
1 In this administration two versions of the probe were used, reversing the way comparisons were worded – e.g., whether students were asked to compare Na+ with Na, or Na with Na+. But, this did not seem to make any difference to the general pattern of responses.
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