Scientists throughout history have generally been loathe to accept a new theory that doesn’t fit with existing theory until it’s forced upon them with a preponderance of evidence. Correct me if I’m wrong on any of my facts, but it seems to me that Rutherford discovered one property of the atom while violating two existing concepts. He created a model wherein all the positive charges, which should repel each other, were attracted in the middle of the atom, while charges that should be attracted to the nucleus were repelled (perhaps Rutherford assumed the speed of the electron offset the charge attraction like a planetary orbit?).
Based on my brief reading on the gold foil experiment, all he discovered was that there was a lot of space either within, or between atoms. Why did he make the assumption the space was within? And it seems there are many ways to construct or conceive of an atom with a lot of internal space that doesn’t require like charges to be concentrated in the middle. How did he come up with the right one? I assume I’m missing some finding in the experiment. Thanks in advance for your help.
If I could just add a second question - How did Mosley, Rutherford, et al explain away the difference between the number of protons and the atomic weight before the neutron was discovered?
The charge to mass ratio of the electron was known, so it was clear that electrons were light.
The deflections were caused by something heavy - since there was not much recoil by being hit by an alpha particle, and something positively charged (since the positively charged alpha particles were deflected).
The space between atoms can’t be too large, since gold is a conductor - so electrons travel freely from atom to atom (which also suggests that they are not tightly coupled with the nucleus)
So as a first guess, a heavy positive nucleus with the electrons doing something somewhere well away makes sense.
Someone with more information will be in shortly with more, I hope.
A brief timeline as I understand it:
[ul]
[li]1911: Rutherford proposes his model for the atom, with light electrons around a small, heavy, positively-charged nucleus. The magnitude of the charge seemed to be roughly 1/2 the atomic mass.[/li][li]1911: Soon thereafter, Antonius van den Broek notices that the charge of the nucleus in Rutherford’s model is equal to the atom’s atomic number.[/li][li]1913: Mosely provides experimental confirmation that nuclear charge = atomic number. At this point there’s no concern about missing atomic mass because the proton hadn’t been discovered yet – we know that a nucleus has a charge equal to its atomic number, and that the charge is roughly 1/2 the atomic mass, but we don’t know that the nucleus with charge A contains A protons.[/li][li]1919: Rutherford discovers the proton which has atomic mass 1. Now you have to worry about missing atomic mass. [/li][li]1920: Rutherford proposes the existence of the neutron. He didn’t get it quite right, but he had the basic idea of neutrally-charged particles in the nucleus.[/li][/ul]
I think that’s the basic gist, anyway.
Your reading is very wrong. Andy L has listed what he actually discovered in the experiment, but I’d like to repeat it and include what Rutherford’s observations were.
Rutherford shot alpha-particles at gold foil. He’d previously shown that alpha-particles were positively charged and were in fact helium ions, whatever a helium ion actually was. According to the prevailing theory at the time, neutral atoms were neutral also at quite small scales, so the positively charged alpha-particles should be deflected little or not at all.
Instead they showed a certain mix of no deflection, lots of deflection and bouncing practically straight back, which showed that the positive charge was very concentrated, and that this concentrated positive charge had a good deal of the mass of the atom. The combined knowledge then of the negatively charged electron which carried little of the atomic mass, the positively charged alpha-particle that carried the positive charge and most of the mass of the helium atom, and the gold foil having small, but heavy, concentrations of positive charge naturally led to the hypothesis of a positive nucleus with negative electrons orbiting at a distance.
Dude was wicked smart and the model was the best he could come up with to explain his experimental observations.
He was shocked to discover some alpha particles being bounced off the foil ( hence the famous ’ tissue paper and artillery shells’ quote) and a small, dense nucleus was the clearest explanation as AndyL said.
I would have started with 1904: Thompson proposes the plum pudding model.
Generally a sparse “pudding” of electrons with a few hard bits of “plums”/positive charges scattered about.
The OP is also asking about the issues of why don’t the protons repel each other* and why don’t the electrons get drawn into the protons?
At the outset these were known issues but became resolved over time. That’s just how Science works. You first figure out how certain things work even though there seems to be problems, but you presume that those will be worked out later.
It’s like today with gravity and galaxies. We know there’s something odd going on with the rotation rates. But we’re not going to chuck the whole theory of gravity. It’s just going to take some time to figure out what’s going on, e.g. dark matter.
This point was made in one of the infamous Jack Chick tracts I saw. Ergo all of Science is wrong and therefore Jesus is right! Such logic. Later the tract was revised with that part changed to make fun of gluons. Apparently even Chick had a lower limit on stupidity if people mocked him enough.
Basically, we can compute how positively-charged particles would be deflected by a charged (positive or negative), stationary point particle. Basically a scattering likelihood proportional to 1/sin^4(theta/2) where theta is the angle of deflection, and Rutherford observed deflections like that.
That implied that the scattering was caused a charged particle that was both compact compared to the interaction distance (so the alpha particles would “see” it as a point particle) and heavy (so the interaction would be mostly the alpha particles being deflected, not the target being pushed out of the way by the alpha particles). It was already known that electrons were a part of matter, but they were much lighter than alpha particles, so they could not be the deflection centres, so it made sense that those centres were the positively-charged part component of matter.
And once you have a heavy, positively-charged particle, and some light, negatively-charged particles, the idea that the latter is just orbiting the former like a little solar system is probably the simplest configuration that makes sense.
I’m loathe to search for it or link to it, but I recall whichever nuclear force that is involved here being dismissed. It’s the love of Jesus that holds the nucleus together.
I’m gobsmacked. So atomic numbers came before the discovery of the proton. This was all new to me, or I’d never thought about it before. According to wikipedia, Mendeleev, when he was passing out atomic numbers, started with the ordinal numbers from the list of the atoms sorted by atomic weight. Then he swapped some elements round in his table to better line up properties with position.
Yep. Atomic number preceded the discovery of any subatomic particle (the first one discovered was the electron - in 1897). Atomic number preceded the acceptance of atoms as a real thing (Mach didn’t believe that atoms were real - in the 20th century).
On a related question (at least in my mind), when Cockcroft and Watson split the lithium atom did they have any prediction of the resulting elements and whether the process would be exothermic?
Whoops missed the edit window. By resulting elements I mean whether it would be all hydrogen, a helium and hydrogen, 2 helium, etc. Did they know the proton would integrate into another atom or just provide the energy to split the lithium?
Here’s their preliminary letters to Nature. So they suspected that they were producing Lithium-8 which decayed to two alpha particles. Not sure if they would have known about the instability of Lithium-8 and it’s decay particles. Given the abundance of Lithium 6 and7 naturally but not other isotopes, one could have guessed it wasn’t stable. Cautious folk so they did a bunch of analysis to verify it for their final results.