B Why did Rutherford conclude the atom was mostly empty space?

[rocknrollkieran]

TL;DR Summary

Why did Rutherford conclude the atom was mostly empty space after firing alpha particles at gold leaf?

Why did Rutherford conclude the atom was mostly empty space after firing alpha particles at gold leaf?

Was it not equally possible that the observed behaviour of the alpha particles was due to space between the gold atoms rather than space inside the gold atoms?

 

[tech99]

I suppose the fact that gold is a solid, a good conductor and is not usually transparent indicates that the atoms are very closely packed.

 

[Ibix]

I believe they also knew the typical energies needed for ionisation. If the nucleus is a point charge that sets limits on how close the electrons can be.

 

[PeroK]

TL;DR Summary: Why did Rutherford conclude the atom was mostly empty space after firing alpha particles at gold leaf?

Why did Rutherford conclude the atom was mostly empty space after firing alpha particles at gold leaf?

Was it not equally possible that the observed behaviour of the alpha particles was due to space between the gold atoms rather than space inside the gold atoms?

The conclusion was that all of an atom's positive charge and most of its mass was concentrated at the nucleus.

 

[rocknrollkieran]

I'm not sure if gold's conductance is relevant as this is due to the loosely bound electrons in the outer shell which hardly interact with the alpha particles.
Gold is very malleable so can be beaten into very thin sheets. Rutherford seems to have concluded that the gaps between the atoms were much smaller than the gaps between the nucleus and electrons within the atoms. I don't see how he could have safely arrived at this conclusion.

 

[rocknrollkieran]

The conclusion was that all of an atom's positive charge and most of its mass was concentrated at the nucleus.

Yes due to a small percentage of the alpha particles being deflected with the majority passing through.
But how could he conclude that the alpha particles were passing through the atoms rather than between them?

 

[Frabjous]

You are getting things backwards. It was the large scattering events that mattered, not the ones that got through. A nucleus explains the large scattering events.

 

[Ibix]

But how could he conclude that the alpha particles were passing through the atoms rather than between them?

There were already models of atom size which gave much larger measures than Rutherford's estimate of the nuclear size. van der Waals got the 1910 Nobel Prize for one, for example.

 

[PeroK]

Yes due to a small percentage of the alpha particles being deflected with the majority passing through.
But how could he conclude that the alpha particles were passing through the atoms rather than between them?

He didn't. If an alpha particle passed through the gold leaf, it could equally have passed between atoms. However, his atomic model had the negative electrons scattered around the nucleus.

In any case, the gold leaf itself must be mostly space - which can be calculated from its area and overall mass.

 

[Baluncore]

But how could he conclude that the alpha particles were passing through the atoms rather than between them?

The electrons are irrelevant. Through the atoms is the same as between the nuclei.

The ionic radius of gold can be computed from the density and the crystal structure. The ionic radius is half the distance between two bonded nuclei. The volume between the nuclei contains only electrons and ionic bonds. Alpha particles were most often passing through that space without colliding with the nuclei. The conclusion must be that the nuclei are compact structures. Electrons are not an obstruction, as they can always be shared or be swept out of the way by an alpha particle.

 

[Ibix]

Another thing to think about small atoms with large spaces is where is the negative charge in this? Rutherford's analysis is just Keplerian orbits, which means that there has to be a more-or-less point charge. If you have a more-or-less point atom then the backscatter would look different - you might get bounce back from direct hits, but I don't see why you'd get any deflection from passing between neutral atoms.

At the least, if Rutherford wanted to propose small neutral atoms and deflection then he'd need to propose a mechanism for the latter. But point positive charges and some kind of diffuse low density negative cloud does the job, and is actually not that far from a mirror image of the then-popular plum pudding model.

 

[rocknrollkieran]

The electrons are irrelevant. Through the atoms is the same as between the nuclei.

The ionic radius of gold can be computed from the density and the crystal structure. The ionic radius is half the distance between two bonded nuclei. The volume between the nuclei contains only electrons and ionic bonds. Alpha particles were most often passing through that space without colliding with the nuclei. The conclusion must be that the nuclei are compact structures. Electrons are not an obstruction, as they can always be shared or be swept out of the way by an alpha particle.

So what you're saying is that even if the gold foil is beaten down so it's only 1 atom thick, there is no space between the atoms in the metallic structure - all of the space is between the nuclei and the surrounding sea of shared electrons?

 

[PeroK]

The electrons are irrelevant. Through the atoms is the same as between the nuclei.

The ionic radius of gold can be computed from the density and the crystal structure. The ionic radius is half the distance between two bonded nuclei. The volume between the nuclei contains only electrons and ionic bonds.

Would Rutherford have known this?

 

[Baluncore]

... , there is no space between the atoms in the metallic structure - all of the space is between the nuclei and the surrounding sea of shared electrons?

No, I am saying all the space between the nuclei contains electrons, and electrons do not significantly impede alpha particles.

Would Rutherford have known this?

I don't know what he "knew" or guessed.
Is this thread discussing Rutherford's model, or a more recent model, in the light of Rutherford's model?

Why did Rutherford conclude the atom was mostly empty space after firing alpha particles at gold leaf?

Was it not equally possible that the observed behaviour of the alpha particles was due to space between the gold atoms rather than space inside the gold atoms?

The atoms must be somehow bonded to form a solid. What makes that bond? How do you then differentiate between; the space between the gold atoms, and the space inside the gold atoms, (but outside the nucleus).

 

[PeroK]

I don't know what he "knew" or guessed.
Is this thread discussing Rutherford's model, or a more recent model, in the light of Rutherford's model?

That's why these historical questions get messy. What exactly did Rutherford know and how much was he guessing?

 

[Baluncore]

That's why these historical questions get messy.

Rutherford 1911, Bohr 1913, and many others employed the solar system model with electrons orbiting the nucleus, like the rings of Saturn, or like planets orbiting the Sun. Those models lasted from before Thomson's plum pudding model, until to the quantum model.

Rutherford's nuclear model did not involve electron scattering, nor explain chemical bonding. Ionic bonding by shared electrons was being developed by chemists, in parallel with, Rutherford's and Bohr's models.

 

[Dale]

But how could he conclude that the alpha particles were passing through the atoms rather than between them?

Did he actually conclude that?

 

[Steve4Physics]

But how could he conclude that the alpha particles were passing through the atoms rather than between them?

The experimental results alone didn't lead to that conclusion. The results indicated that there were tiny massive charges (which could significantly deflect a small proportion of the alpha particles) with large gaps between them. That's all.

But additional knowledge/information was available at the time, e.g.
- that atoms exist and are (generally) neutral;
- the approximate size of atoms;
- that atoms contain electrons which have small mass.

Put it all together and you get a bigger picture.

 

[Nugatory]

Why did Rutherford conclude the atom was mostly empty space after firing alpha particles at gold leaf?

The recoil pattern that he observed indicated that essentially all of the mass is concentrated in tiny positive-charged lumps. The electrons have to be somewhere, that somewhere has to be reasonably far from the nuclei (because otherwise their negative charge would cancel the positive charge of the nuclei), therefore there has to be a lot of empty space between nuclei and electrons.

 

[Andrew Mason]

Rutherford would have known how many atoms were in a mole of gold using Avogadro's number. That number had been accurately determined from Milliken's oil drop experiment to determine the charge of an electron compared to a Faraday, which is the charge of a mole of electrons. So he would have been able to determine the number of atoms per unit of area of his uniformly thin gold foil and, therefore, the distance between the centres of the gold atoms.

Due to the magnitude of forces required to compress substances other than gases, including gold, it could be shown that the distance between atoms could not be accounted for by the Coulomb repulsion between the positive nuclei in atoms. So it must be accounted for in the Coulomb repulsion between the negative charges (electrons) in the atom which must be at a much closer distance than the space between nuclei.

AM

 

[Paul Colby]

Rutherford measured the differential cross section. Its main feature, what most alphas did, is described by coulomb scattering, which had (has) a well understood model prediction. The coulomb cross section is strongly peaked at small scattering angles and falls off for higher angles. The smaller backward peak he found means some of the incident alphas were bouncing off something that had to have the mass of a gold atom.

 

[rocknrollkieran]

The atoms must be somehow bonded to form a solid. What makes that bond? How do you then differentiate between; the space between the gold atoms, and the space inside the gold atoms, (but outside the nucleus).

So are there areas between the gold atoms where there is zero probability of finding an electron?

 

[PeroK]

So are there areas between the gold atoms where there is zero probability of finding an electron?

Rutherford didn't know anything about probabilities. That came later.

 

[Steve4Physics]

So are there areas between the gold atoms where there is zero probability of finding an electron?

No. Oversimplifying a bit...

In solids and liquids, atoms are effectively touching each other.

In (sufficiently low pressure) gases, there are relatively large gaps between atoms.

I think the above was generally accepted knowledge in Rutherford's time.

 

[Baluncore]

So are there areas between the gold atoms where there is zero probability of finding an electron?

That depends on what you mean by "between the gold atoms". You seem to have an undefined model in your head that gives atoms insides and outsides, even though the atoms are firmly held in place, within the crystal structure, by ionic chemical bonds.

As particles, if the gold is conductive and solid, there will be free electrons, statistically everywhere, and there will be shared bonding electrons, holding the nuclei in place.

But when you look at it from the quantum model, all is determined by exclusions, with wave equations filling the space, probably.

At some depth of detail, you should grant your model, the freedom to become physically indeterminate, and let certainty go. Read more of "Alice's Adventures in Wonderland" from 1865, with quotes like "Why sometimes I believed as many as six impossible things before breakfast".

Rutherford was born in 1871, I wonder if, or how, Alice influenced him and his contemporaries.

 

[Nugatory]

So are there areas between the gold atoms where there is zero probability of finding an electron?

No, but does it matter? An alpha particle follows pretty much the same trajectory through a region that contains electrons and a region that doesn’t.

 

[Herman Trivilino]

TL;DR Summary: Why did Rutherford conclude the atom was mostly empty space after firing alpha particles at gold leaf?

Was it not equally possible that the observed behaviour of the alpha particles was due to space between the gold atoms rather than space inside the gold atoms?

Space between the atoms, space inside the atoms? I think you are just arguing semantics here. Atoms don't have a defined edge. Alpha particles were deflected by different amounts. The ones that were deflected the most were closer to the gold nucleus, the ones that were deflected the least were further from the nucleus. So the relevance is in the alpha particle count. Lots more alpha particles were deflected by smaller amounts, very few were deflected by larger amounts.

Thus he concluded that the gold nuclei were distributed in such a way that there was lots of space between the nuclei.

Whether you call that space within or between the atoms is not relevant. What's relevant is that most of the mass of the gold atom, by far, is concentrated in a tiny space called the nucleus.

The competing theory at the time was that the mass was more or less evenly distributed through all of the atom, and indeed throughout all of the gold foil leaf. Rutherford put that idea to bed. That is the great idea for which he is credited.

 

[Pisica]

TL;DR Summary: Why did Rutherford conclude the atom was mostly empty space after firing alpha particles at gold leaf?

Why did Rutherford conclude the atom was mostly empty space after firing alpha particles at gold leaf?

Was it not equally possible that the observed behaviour of the alpha particles was due to space between the gold atoms rather than space inside the gold atoms?

If the atoms were far from each other with a lot of empty space between them, then it would be gas, we could apply pressure and bring them closer/

But if it cannot be compressed, it is liquid or solid, that is, the atoms are as close as possible to each other.