A topic in Learners’ conceptions and thinking
The assumption of initial atomicity is a common alternative conception founds among school-age science learners.
(Read about alternative conceptions)
It is the [false] assumption that in chemical processes, such as chemical reactions, the reactants (the substances present before reaction) are in the form of discrete atoms. This may be something a learner explicitly thinks will be the case, and deliberately applies; or it may be a tacit assumption that they do not think to question.
What is wrong with the assumption of initial atomicity?
The difficulty with this conception is not that substances in the form of atoms would not react, as they usually would, but just that very few substances exists as atoms under common conditions. Atomic materials are usually too labile to remain in that state.
Under familiar conditions, most substances would be in the form of molecules (water, sulphur, oxygen, methane…) or arrangements of ions where the charges of cations and anions balance (sodium chloride and other salts) or arrays of ions associated with electrons that are no longer firmly associated with particular atoms, but are still bound to the wider array (that is, metals).
There are more complex arrangements, such as in copper sulphate crystals which has a structure including metal (copper) cations, complex anions (the SO42- ion), and water molecules. However, very few common substances exist at familiar conditions as discrete atoms.
The only common exceptions are the noble gases: helium, neon, argon, xenon, krypton and radon. These exist at room temperature as gases comprised of discrete atoms (which are sometimes considered as single atom molecules). However, the noble gases have limited chemistry – they generally do not react very readily, or indeed at all with most other substances. (Indeed, the noble gases used to be called the 'inert' gases, as it was for a long time thought they did not ever react with anything.)
So, generally, reactions are not atomic. Although it is sometime possible to atomise substances – to convert a sample into atoms by heating, as in the mass spectrometer for example – this requires energy to break up the bonds in the substance. The starting materials themselves will almost never (unless you are reacting xenon, say) exist as separate individual atoms.
Why is this such a serious alternative conception?
The assumption of initial atomicity is important, as it has consequences for learning chemistry. One of the most common alternative conceptions in learning chemistry is the full outer shells explanatory principle which is the idea that chemical reactions occur so that atoms can fill their shells. This conception is associated with a wider framework of common alternative conceptions (the octet framework), and is not only pervasive, but is an idea that, once students have adopted it, is very hard to put aside. Even when learners have learnt scientific models of why reactions occur (for example, from thermodynamic considerations) there is a strong tendency to slip back to explain chemistry in terms of atoms seeking full shells or octets of electrons.
(Read more about 'the full outer shells explanatory principle')
(Read more about 'the octet framework')
Yet, the full outer shells explanatory principle actually make very little sense as an explanation for reactions, as none of the reactions usually met in school science involve substance that are atomic, so explaining chemical reactions using the the full outer shells explanatory principle is a bit like explaining that dogs can run faster than centipedes because dogs have more legs than centipedes – that is, an explanation that anyone should be able to dismiss with just a little thought about the premise it is based on!
Why do so many learners adopt the assumption of initial atomicity?
It is not entirely clear why so many students seem to assume that in chemical processes, such as chemical reactions, the reactants (the substances present before reaction) are in the form of discrete atoms. There is probably not a single reason, but here are some suggestions:
- Atoms make a psychologically comfortable starting point for thinking about chemistry. When thinking about atoms, it may intuitively seem that atoms are a good place to start. Chemistry is explained at the submicroscopic level in terms of abstract ideas and undetectable conjectured entities (molecules, ions, electrons etc.). We need a starting point for thinking about this unfamiliar realm, and perhaps atoms seem to provide this as they are simpler than molecules, and are always electrically neutral unlike ions).
- We teach atom first: then again, perhaps it just seems that atoms are a sensible starting point because we usually teach about atoms before ions and molecules. (Or, do we just teach first about atoms because they make a psychologically comfortable starting point – that is perhaps 1 and 2 are mutually reinforcing!)
- Creation myths. Perhaps it seems sensible to start with atoms, because in nature everything started as atoms, and only later did atoms form molecules and ions and… But of course, that is not an explanation of the assumption of initial atomicity, but rather making that assumption.
- In terms of current scientific models this assumption is false. There were no atoms formed in the big bang as it was too hot for atoms to exist. Apart from hydrogen and helium and some lithium, all the other elements have been formed in nucleosynthesis in stars at temperatures where it is much too hot for atoms to exist (n.b., the process is called nucleosynthesis). Does that mean atoms will condense out from the plasmas where these heavier nuclei are formed once the material is at a lower temperature: only if atoms are the most stable systems to form under the conditions. (Yet if atoms were the most stable systems, then they would not tend to go on to react to form molecules etc.)
- But, surely, "everything is made of atoms". Many students learn this motto or slogan or some variant of it. Is it true? It depends what we mean by it. If it means that materials tends to contain submicroscopic systems which can be understood as more or less, basically atoms – that is, structures we can understand as being like atoms joined together or modified in various ways – then it is a reasonable generalisation. But if it means that all materials are comprised of distinct, discrete atoms, or that all materials have been derived from distinct, discrete atoms, then it is simply incorrect. Mottoes can be useful for getting the gist of key ideas across to students, but then are likely to be adopted as slogans rather as starting points for more sophisticated thinking.
- Linking atomicity with elements. Sometimes learners know that most substances are not atomic. Compounds may be accepted as molecular, but elements are still thought of as atomic as if there is a common pattern between the macroscopic and submicroscopic levels: atom-element as molecule-compound. Unfortunately for learners chemistry is more subtle than that!
- Teaching representations. A common type of representation of chemical reactions at the submicroscopic level shows the reactants as atomic. These figures are easily found on the internet and in many school level textbooks. It is interesting to ask why: do authors and teachers think this helpfully simplifies the science for the learners – or is it possible that the authors and teachers themselves make (if only at a tacit level without noticing) the assumption of initial atomicity?
