Keith S. Taber
Brian was a participant in the Understanding Chemical Bonding project. He was interviewed during the first year of his college 'A level' course (equivalent to Y12 of the English school system). Brian was shown, and asked about, a sequence of images representing atoms, molecules and other sub-microscopic structures of the kinds commonly used in chemistry teaching. He was shown a simple representation of an atom which he identified as showing "electron configuration…of an element, sodium".
Brian identified the electrons and nucleus, and was asked about the arrangement of the electrons:
Can you tell me why the electrons stay there, in these positions, why they don't fly off into space?
'Cause they're held by the nucleus.
In what way does the nucleus hold them, any idea?
It's got a positive charge, and so attracts the electrons, which are negatively charged.
Okay, so, it's got an electrical attraction there.
Yeah.
Why don't they just go into the nucleus then, if they're attracted, why don't they just get pulled into the nucleus?
Because, 'cause there's more than one electron, they repel each other, and keep them out.
Ah, so what about these ones [on opposite sides of the nucleus] though, these repel each other do they, even though they
Yeah.
are drawn on opposite sides?
Yeah.
So that's what stops them actually falling into the nucleus, that they repel each other?
Yeah.
It seems that Brian recognised electrical interaction between the nucleus and the electrons in an atomic structure. He also recognised that electrons would repel each other, but did not seem to have considered that in itself that was an insufficient explanation for the structure of the atom (as, for example, the sole electron in a hydrogen atom does not fall into the nucleus).
Although Brian's explanation was based on sound principles (negative electrons repel each other), it is an alternative conception. Coulombic forces are proportional to charges and diminish with separation – inspection of the figure should suggest that the two inner electrons (tending to be pushed inwards by outer electrons) at least must experience net force towards the nucleus.
The stability of atoms – the failure of electrons to spiral into the nucleus leading to atoms collapsing – was one of the phenomena which led to the development of quantum theory. In classical physics the stability of electron orbits was a puzzle to be solved, as orbiting electrons 'should' have acted as electrical oscillators, and emitted energy as their orbits decayed into the nucleus whilst the atom (very quickly) collapsed. Quantum theory posited limited allowed energy states, rather than a continuum of possibilities – but learners new to the topic do not know about this.
Often learners simply accept atomic structure when presented with planetary-system type representations of the atom. 'Quanticles' such as atoms are so far from direct human experience that they presumably seem strange enough such that questions that might seem obvious to a teacher do not arise for students. (Students also commonly accept the 'atom is like a tiny solar system' teaching analogy, and may map inappropriately between the two systems.)
