Conceptions of atoms

Alternative conceptions of the atom

A topic in science concepts and learners' conceptions and thinking

Science teaching involves teaching scientific concepts, yet these may be difficult to completely characterise, and may prove difficult for many learners to master (a very brief introduction to why this might be can be found at the foot of this page).

This page introduces some issues related to atoms, with links to examples discussed in more detail elsewhere on the site.

Atoms

For much of its conceptual history, the atom was considered hypothetical.

Some (but certainly not all) of the ancient 'Greek' philosophers considered that matter was made of atoms. However this atom was 'metaphysical' in the sense that it was an idea that was part of a philosophical system in the rationalist tradition. (In other words, based on thinking about what the nature of the world is, but without resorting to testing ideas empirically as in the modern scientific tradition.)

The idea of the atom was adopted in early modern chemistry (e.g., by Dalton) where it was used as an explanatory device in relation to actual laboratory observations. Despite this, the question of whether atoms 'really' existed or were just a useful way of thinking (a kind of heuristic device) remained uncertain into the twentieth century.

The atoms of Democritus

Democritus (c. 460 – c. 370 BC) proposed that everything is composed of atoms. His atoms, were atomos, indivisible. They were indestructible, and came in a range of shapes and sizes.

The ancient idea of atoms assumed they can in various shapes (Image by Peter Lomas from Pixabay)

Thus these atoms are quite unlike those of modern science. Yet, something of this idea of indivisible atoms seems to often influence learners when they first meet the topic (Taber, 2003) . Democritus's atoms can be considered to be basically tiny solid particles – whereas atoms (see below) do not have distinct surfaces

Atoms or molecules

Although Dalton conjectured substances may be comprised of atoms, the early use of the concept in chemistry did not distinguish between what would today be considered 'atoms' versus 'molecules'. So often references to atoms in early chemical writing might better be understood in today's terms as molecules.

Misleading atomic slogans

It is commonly said that "everything is made of atoms", yet of course, that cannot literally be true or one gets an infinite regress. Yet learners may take such mottoes very literally (e.g., A chemical bond would have to be made of atoms). Similarly, the idea that 'an atom is the smallest amount of matter you can get' led to one learner suggesting that a nucleus by itself would not count as matter.

Atomic structure

Given the scale of atoms they are not quite like familiar particles. Some learenrs struggle to appreciate the scales involved.

Read 'The cell nucleus is probably bigger than an atomic nucleus'

In some ways atoms sometimes behave like tiny bearing balls or marbles – but not always. They are so small that quantum effects are significant – in some ways atoms have wavelike properties. They might better be considered something other than particles or waves – quanticles.

Atoms do not have definitive surfaces (Image by Anand Kumar from Pixabay)

For example, atoms do not have a definitive surface, so do not have a definite volume or diameter/radius. Rather they are like little balls of fuzzy fields which drop in magnitude with distance.

When atoms are modelled with physical spheres (such as polystyrene spheres) this is not obvious, and students may assume atoms are like familiar spheres, but just very much smaller.

It perhaps does not help that scientists and sometimes science teachers (and textbook authors) do talk about atomic size, atomic radius, and atomic volume. This is possible by using a convention where an arbitrary boundary is imagined that contains a volume where the atomic electrons will be found most (say, 95%) of the time. This is a bit like imagining that the earth's atmosphere has a sudden edge by deciding that when the air gets to some chosen degree of rarity that we will no longer count it as part of the atmosphere.

The nucleus is at the centre of the atom and takes up a tiny proportion of the nominal volume. Most physical models and graphical representations show the nucleus far larger than it would be – as if an atom was drawn to scale the nucleus would be too small to be seen – or the drawing would be too large to fit in the room! Students may not always appreciate this and may assume that models and figures are meant to be to scale rather than schematic.

The atom is not like a tiny solar system: discuss

A common comparison is to suggest that the atom is like a tiny solar system. This works in relation to most of the mass being at the centre in both cases. However, it is a limited comparison in other regards. (The solar system is largely concentrated around the plane of the eclectic; planets have their own moons; planets differ in mass…)

The current state of scientific knowledge about atoms would probably suggest that the only models that are fully satisfactory are mathematical models. Despite this, physical and visual representations (inevitably to some degree simplifications or distortions) are very useful and are presented in education.

The model of the atom as a central nucleus with a number of orbiting electrons does offers a useful basic analogy with the solar system, as long as differences as well as similarities are explored.

There are challenges here, though. A force is needed to hold an object in orbit about an another – although learners may not appreciate this even if they have learnt about Newton's laws. This reflects a historical idea that was prevalent over many centuries that in the heavens circular motion was 'natural', that is, it did not require any explanation.

In the case of the solar system, the centripetal force is provided by gravity (although in a relativistic account this is replaced by the notion of the geometry of space time being distorted by mass – in effect, in Einstein's model, we might suggest orbital motion is natural!) It may surprise teachers, but it is not unusual for learners to suggest that the force acting in the atom is also gravitational – perhaps suggesting that the analogy here may be unhelpful in introducing atomic structure (Taber, 2013).

Shells

The introductory model of atoms often taught in schools considered electron configuration in terms of shells (K, L, M…). This model is a simplification, so, for example, it ignores how electronic orbitals from 'different shells' interpenetrate.

The ideas of shells will be familiar to learners from gastropods such as snails and other creatures that have a hard protective shell. It is not unknown for learners to therefore assume that the atomic shells are similarly hard: that htey are actual parts of the atom on which electrons are located. (One might suggest analogous to the hard crystalline spheres that were once thought to carry the planets in their journeys around the solar system!)

Orbitals

The orbital model of the atom is abstract and complex. Indeed, the necessary equations have only been solved precisely for single electron (hydrogenic) systems. The common ways of labelling orbitals (1s 2s 2p 3s…) strictly only applies to such systems.

Students who have learnt the shells model of the atom may assume orbitals are synonyms for shells ("orbitals became confused with shells, sub-shells, energy levels and visual representations of probability envelopes", Taber 2002a)

In simple LCAO (linear combination of atomic orbital) models students may not appreciate that the atomic orbitals are considered to have ceased to be present when they overlap to form molecular orbitals (Taber, 2002b). The designation of hybridisation (e.g., sp2) assumed for an atomic orbitals that overlaps in bond formation may be assumed to apply to the resulting molecular orbital.

Stability of atoms

Whether atomic systems can be considered stable or not depends upon the environment – the chemical context. A neutral atom has a balance of positive and negative charge, and so would be stable if isolated.

Yet because chemistry education often puts a strong emphasis on the relative stability of electronic structures of noble gases, learners often assume that the atoms of other elements are intrinsically unstable.

Read about a classroom resource to diagnose this conception

Students may assume that an ion with a full outer shell or octet of electrons will be more stable than the neutral atom, even if that ion has a large (e.g., C4+) and/or inappropriately signed (Na7-) charge (Taber, 2009).

Read about a classroom resource to identify which ions students assume are stable

Read more about learner conceptions of chemical stability

Work cited:

Why is teaching and learning of science concepts so difficult: a brief overview

There are many possible ways of conceptualising natural phenomena and science topics. Arguably, we each have somewhat unique and idiosyncratic takes on scientific concepts, so there are always alternative conceptions which overlap and match to varying degrees.

Students often have intuitive ideas about the natural world, or have come across 'folk ideas', which are not consistent with scientific concepts. Some science teaching is regularly misunderstood so misconceptions circulate among students (and may even be found represented in text books).

Scientific theories may be sophisticated and nuanced, and so neither suitable for teaching novice learners nor (even with more advanced learners) for introducing in one step. Science curricula often have simplified and approximate representations of current scientific ideas ('curricula models') set out as target knowledge – so accounts at different levels are not entirely consistent.

Scientific ideas have developed over time, and have become more sophisticated and finely detailed. Many ideas once found useful in science have fallen into disuse either because it is now thought they were actually wrong, or they are no longer seen as helpful ways of thinking about a topic. Many of these ideas that are less developed, out of date, or now discredited, remain as 'conceptual fossils' that can be found in some science texts or more general literature and discourse.

Scientists develop models as thinking tools, often knowing the models are flawed: perhaps only giving approximate outcomes, or only being applicable in limited ranges of situations or they may be purely hypothetical (to be used to think through what would happen if…) without any expectation they reflect nature. Sometimes such models may appear in books to be of similar status to well founded principles laws and theories. Theories themselves vary in the extent to which they are considered to be likely correct, or just a provisional thinking tool.

Scientists, science communicators and journalists, and science teachers, use various techniques to help get a cross novel or abstract ideas: these can include analogies, similes, metaphors, narratives, and so forth. Sometimes these ideas get repeated and used so often they may seem to be part of the scientific idea, rather than just a linguistic tool. Indeed, sometimes terms originally used metaphorically become so widely adopted that they take on a new scientific meaning somewhat different form the original meaning.

Given all of that, it is not surprising that learning and teaching science can be quite challenging!

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