Atoms, elements and compounds · Model
The atom: Dalton's model
People tried to turn lead into gold for fifteen hundred years and never once managed it. What would have to be true for that to be impossible?
Start here
Fifteen centuries of failure is a result.
Alchemists were not fools. They were careful, they kept records, and they had furnaces, acids and time. They dissolved lead, burned it, mixed it with everything available — and not one of them ever produced a single grain of gold.
What would explain a failure that complete?
Lead is built from one kind of particle and gold from another, and nothing you can do in a flask changes one kind into the other. In 1803 John Dalton wrote that idea down properly, and modern chemistry starts there.
You already have a particle model: everything is made of particles, too small to see, always moving. It explained melting, pressure and diffusion. It said nothing at all about there being different kinds of particle — and without that, nothing in chemistry works.
The model · switch a claim off
Dalton made three claims. Take one away.
All three claims on
A model earns its place by explaining things that were already known. Switch a claim off and watch which observations it takes down with it.
Commit first. Which claim do you think the other two could not manage without?
Dalton's claims — tap to switch one off
Things known before Dalton wrote
No amount of heating, dissolving or burning ever turned lead into gold.
If atoms could change kind or be taken apart, there is no reason this should never have worked.
explained
Water from a river, from the sea and from a laboratory always contains exactly the same proportion of hydrogen to oxygen by mass.
Without identical atoms joining in fixed ratios, the proportions would come out differently every time.
explained
Seal a reaction in a flask and its mass is exactly the same afterwards as before.
If atoms could be destroyed or made, mass would drift up or down and nobody would expect it to balance.
explained
Copper and oxygen combine to make one compound in one ratio, and a second compound in another — but never anything in between.
Without whole-number ratios there is nothing to stop every proportion in between existing too.
explained
All three claims on, and every observation is accounted for. That is what a model is for: not being true, but paying its way.
Key fact
An atom is the smallest particle of an element. There are about a hundred kinds, and no chemical reaction turns one kind into another.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
How small · zoom until you find one
Small is not a strong enough word
1 of 5 steps
Start at a centimetre of copper wire and go closer. Watch the scale on each step: the jumps get bigger as you go down, because the distance between what a lens can show you and the size of an atom is enormous.
1 cm across
A centimetre of wire. Solid, orange, bendable — everything you know about copper is true at this size.
Think again
“A copper atom is a tiny bit of copper — orange, shiny, and it conducts electricity.”
Copper is orange, shiny, bendable and conducts. Commit to how much of that a single copper atom has before you read on.
None of it. Colour is what happens when light meets a huge number of atoms together. Conducting is something that only makes sense when there is somewhere for the charge to go. Bending needs layers of atoms to slide over each other, and one atom has no layers.
The properties of a substance belong to the crowd, not to the individual. What a copper atom has is a kind — it is a copper atom rather than a lead one — and a mass. Everything else about copper emerges when you put enough of them together.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What is an atom?
Rung 2 · The one that catches people
Two of Dalton’s three claims are now known to be wrong. Why is his model still taught and still used?
Rung 3 · Explain
Explain why the failure of alchemy is evidence for Dalton’s model, and say what alchemists would have had to be able to do for the model to be wrong.
Rung 4 · Take it somewhere new
A student says: "Dalton was wrong about atoms being unsplittable, so his model should be thrown away." Reply to them, using one thing the model still explains and one thing it cannot.
Key note
Dalton: everything is made of atoms; all atoms of one element are the same and different from every other element's; atoms are rearranged in reactions, never created, destroyed or changed into another kind. Two of those turned out to be not quite true, and the model is still the one you use.
Going further
Dalton said atoms cannot be split. A century later J. J. Thomson knocked pieces off them and found electrons, and it turned out that atoms of the same element can have different masses. Both of Dalton's claims were wrong. The model did not get thrown away, because everything it was built to explain is still explained by it — what happened is that it got a boundary: chemistry never splits an atom or turns one element into another, and that is exactly the range Dalton was working in. Being wrong in a way you can draw a line around is how most good models end up.
Before this lesson
Next in this unit
At GCSE this becomes
- The nuclear atom, electron shells, and the experiments that forced each change.
Where to next
- Next: Elements
- Previous: Testing the model: does it explain everything?
Particles and their behaviour
Ask Mr Badmus AI
Not sure why a model can be wrong and still be used?
Lesson content © MrBadmusAI.