Keith S. Taber
Bill was a participant in the Understanding Science Project. Bill was explaining that he had been learning about the states of matter, and introduced the notion of there being particles:
So how do you know if something is a solid, a liquid or a gas?
Well, solids they stay same shape and their particles only move a tiny bit
So what are these particles then?
Erm, they're the bits that make it what it is, I think.
Ah. So are there any solids round here?:
Yeah, this table.
That's a solid, is it?:
Yeah
Technically the terms solid, liquid and gas refer to samples of substances and not objects. From a chemical perspective a table is not solid. However, I continued, accepting Bill's suggestion of a table being solid as a reasonable example.
Okay. So is that made of particles?
Yeah. You can't see them.
No I can't!
'cause they're very, very tiny.
So if I got a magnifying glass?
No.
No?
No.
What about a microscope?
Yeah.
Yeah?
Probably
Possibly?
Yeah, I haven't tried it.
You haven't tried that yet?
No.
But they are very, very tiny are they?
Yeah.
Bill knew that the particles in a solid were very tiny. He seemed to be convinced of their existence, despite not being able to see them. He considered they were too small to be seem with a magnifying glass, but large enough to probably be seen with a microscope.
Bill, like a good scientist, qualified this answer as he had not actually undertaken the necessary observation to confirm this: but his intuition seemed to be that these particles could not be so small that they would not be visible through a microscope.
Later in the interview, Bill used the term microscopic to describe the particles in a solid, where a scientist would describe them as 'submicroscopic' (or 'nanoscopic'):
Tell me the bit about the solids again? Tell me what you said about the particles in the solids?
They move a very tiny amount, but we can't see that … because they are microscopic.
The term 'particle' used in introductory science classes is often used generically to cover atoms, molecules and ion. These entities are usually much too small to be see with an optical light scope (although other instruments such as scanning tunnelling 'microscopes' provide images showing electric potential profiles that can be interpreted as indicating individual atoms).
Students have no real basis on which to understand the scale of atoms and molecules, and often assume they are particles much like the specks and grains that can just be seen. Bill did not make this error, as later in the interview he told me that "the kind of specks of dust, has lots of particles in it, to make up the shape of it".
This becomes important later because much of chemistry supposes that many of the characteristics of substances as observed in the lab. are emergent properties that results from enormous numbers of molecule-scale 'particles' (or 'quanticles') that themselves have quite different behaviour individually.
Learners however may assume that the properties of the bulk materials are due to the particles having those properties – so students may suggest that, for example, that some particles are softer than others or that in a sponge, the particles are spread out more, so it can absorb more water.
A good way to get over to students the incredibly tinyness of atoms, is to use this example: suppose we wanted to see the atoms in a frozen pea, and could somehow magically expand the pea (by expanding all its constitutent atoms). How large would the pea have to be, before we could see the atoms?
I usually make a 'shaggy dog story' out of this example, by pretending to have a Hollywood-style "Matter-expander" and powerful magnifying class.: "Okay … I turn the Expander Dial up to Position 1, and the pea grows to the size of beach ball. Will that be enough? I take my magnifying glass and look carefully at the surface but … no … as big as a beach ball is not enough … So…. I turn the dial to setting Number 2 … Now the pea grows again, until it's the size of a house. As big as a house, will that be enough? I hold my magnifying glass up … and it's very interesting … but … no atoms … So, on to Setting Number 3 …. "
And only when the pea is as big as the Earth, can we see the atoms, which are about the size of cricket balls and basketballs….
I usually couple this explanation with an explanation for how we use the word "mole" in chemistry: namely, as number-name — like 'dozen' or 'score' or 'ream' — for a number, in the context of chemistry. It's a very large number, because we're dealing with a very large number of things we have to count … because they are so small. (See previous pea-explanation.)
What is that large number? 6.022 x 1 with 23 zeros behind it. Why that strange number? Why not a nice number like 8 x 1 with 24 zeros?
Answer …. but first they have had to learn by heart the word 'nucleon' — which is to proton and neutron as 'sibling' is to 'brother' and 'sister'. (All of these definitions need to be learned by rote, days before the final explanation.) … but if they can just parrot-fashion say, 'A nucleon is either a proton or a neutron', then they can appreciate why a 'mole' is that funny number: because a mole of nucleons masses at one gram. If a nucleon were heavier, or lighter, then 'mole' would be a different number.
So … they have to learn by rote: what is a mole? A really really ginormous number.
What is a nucleon? A proton or neutron.
Why is a mole the funny number that it is? because that many nucleons have a mass of one gram.
There is more to it, of course. The fact that we are just approximating, not really counting; the fact that a neutron has slightly more mass than a proton. Plus the interesting question, how did we choose the mass we did, to be one gram? (Which gives us the chance to bring up the French Revolution, and the general idea of creating the Perfect Rational and Fair Society, and why, although in practice this always leads to guillotines and firing squads, that's not all that it can lead to, and the metric system is an example.)