Keith S. Taber
Annie was a colearner (participant) in the Understanding Chemical Bonding project. In her first interview, during the first year of her two year 'A level' college course, Annie was asked about a (Bohr type) representation of a (sodium) atom. Annie did not know what held the protons and neutrons together in the atomic nucleus, but suggested it might be due to forces from the electrons "pushing":
Interviewer: Can you identify the different parts of that diagram? What's the blob in the centre?
Annie: It's the nucleus.
I: That's the nucleus. Do you know what's in the nucleus?
A: The protons and, no the electrons and the neutrons, no the protons and the neutrons. The electrons are round the outside.
I: There's protons and neutrons in the centre okay.
A: Yeah.
I: Erm, what holds them together, any idea?
A: Is it the forces from the outer ring? Outer rings or outer shells? The electronic forces?
I: What repelling them in? Holding them
A: Yeah.
I: in the centre? It could be.
A: Pushing them.
I: It's not actually, but that's a sensible suggestion. So you haven't actually done anything about what holds the nucleus together?
A: No.
…
The question of why the nucleons should be held together (given the repulsion between positive protons) is not usually considered in school chemistry lesson, and does not seem to be a question which students tend to spontaneously consider. The interview continued…
I: What holds the electrons in place?
(pause, c.4s)
A: Er (pause, c.9s) Not really sure, but I know there's a set pattern of how many can go in each shell, so if its connected with that?
I: Huh hm, do you think, do you think you need anything to hold the electrons in place, or I mean is it just the way the Universe is, or God's will, or, you know, or just aesthetic, you know nature's aesthetic,
A: Yeah.
I: and it looks pretty? I mean do you think there has to be some physical reason why the electrons are there rather than anywhere else?
A: Probably is to do with the structure of it.
I: But you are not, you're not sure why,
A: No.
I: it should be that the electrons should be in orbitals or orbits?
A: No.
I: Rather than just scattered higgledy-piggledy.
A: No, I don't know that.
In this section of the interview, Annie seems to suggest she is not aware of any forces acting on the electrons, and suggests it may be something inherent in the electronic structure which holds the electrons in place. It seems odd that Annie does not invoke a force from the nucleus, given her comment just earlier about a possible pushing from the outer electron ring/shell onto the nucleons. It seems Annie does not know about, or at least does not bring to mind, an electrical force attracting the electrons and nucleus. However, this was tested by a slightly different question…
Okay. So can you tell me why the electrons don't fall out of the atom? I mean if you imagine that this was sort of, er, an atom that's placed vertically, why don't the electron's just fall out of the bottom?
A: The forces hold them together.
I: What kind of forces are they. Do you know?
(pause, c.5s)
A: The attraction from the nucleus, from the protons.
I: So the protons in the nucleus attract the electrons?
A: Yeah.
I: So what kind of attraction is that. What kind of force is that?
A: Er (pause, c.7s) I don't know
So Annie is aware that the electrons are attracted by the nucleus, and specifically by the protons. Despite this, Annie does not suggest the interaction is electronic, or specially refer to charge. Her suggestion that the outer electron shell may push on the nucleus, holding it together, contradicts Newton's third law in that forces between bodies are either attractive or repulsive, not not a mixture of the two. So if the nucleus attracts electrons, then electrons must attract (not push) the nucleus. Annie's suggestion was also inconsistent with the way forces between charges depend upon separation (by an inverse square law): the repulsion between adjacent protons would be far larger than any force due to the more distant electrons.
