By the beginning of the 20th century scientists were only just beginning to probe the mysteries of the atomic world, with the exact nature of these atoms subject to a lot of speculation and theory. Recently [The Action Lab] on YouTube replicated one of the most famous experiments performed at the time, commonly known as Rutherford’s gold-foil experiment.
A part of Rutherford’s scattering experiments, this particular experiment involved shooting alpha particles at a piece of gold foil with the source, foil, and detector placed in a vacuum vessel. Rutherford’s theoretical model of the atom that he developed over the course of these experiments differed from the contemporary Thomson model in that Rutherford’s model postulated that atoms consisted of a single large charged nucleus at the core of the atom, with the electrons spread around it.
As can be seen in the video, the relatively large alpha particles from the Americium-241 source, available from many smoke detectors, will most of the time zip right through the foil, while suffering a pretty major deflection in other times when a nucleus is hit. This is consistent with Rutherford’s model of a small nucleus surrounded by what is effectively mostly just empty space.
While Rutherford used a screen that would light up when hit with alpha particles, this experiment with a Geiger counter is an easy way to replicate the experiment, assuming that you have access to a large enough vacuum chamber.
When a guy is 11-12 orders of magnitude out when he states a number (“10 to the 26th atoms in this bead”), and doesn’t instantly realize it’s unreasonable, it’s hard to trust the rest of what he says or shows…
Yeah, I haven’t been particularly impressed with this Action Lab guy over the years. As far as I can tell, he’s a layman that inexplicably has access to some high-end lab equipment.
Why would he be 11-12 numbers of magnitude out?
Say that bead weighs 1 gram. It would be 4 mmol of Americium.
So that would be 0.004 * 6.023 * 10 to the 26th = 2,47 * 10 to the 21th.
That’s five orders of magnitude out. Still a lot, but it doesn’t discredit the rest of the video in any way.
The guy has a degree in chemical engineering. If he doesn’t have an instant gut feel for what 10^26 atoms (=166 moles) is, his degree is worthless, and so is his credibility as a science presenter. To save you the arithmetic, 166 moles of Am-241 would mass around 166 * 241 (/1000)= 40 kg.
The legal maximum amount of Am-241 in a smoke detector is 1 uCi = 37 kBq, (which is consistent with the 4 k cpm he’s measuring, given solid angle and sensitivity). With the 432 year half life, that’s ~10^15 atoms, maximum.
6.023 X 10^23
“while suffering a pretty major deflection in other times when a nucleus is hit. This is consistent with Rutherford’s model of a small nucleus surrounded by what is effectively mostly just empty space.”
No, that’s not what Rutherford’s scattering model said. It’s not a case where “most of the time you pass straight through, sometimes you go ‘bang’ and bounce backwards.”
Rutherford scattering peaks at 0 degrees, and falls off smoothly but never reaches zero even at 180 degrees. In other words most of the time when it interacts with the nucleus, it’s still just a small deflection. This is because even though the nucleus is small, it’s still positively charged. So you still get interactions even if the alpha particle zips far away from the nucleus.
In fact in Rutherford’s experiments, the alpha particle never actually “hits” the nucleus, because even if the particle is heading square on to the nucleus… you’ve still got a positive charge repelling a positive charge, and that alpha particle does not have enough energy to actually get to the nucleus (if it did, uh, you need a bigger bunker to do that experiment). That’s why Rutherford’s “minimum distance” was way bigger than a gold atom’s nucleus.
The main reason for Rutherford’s experiment really was because Thomson’s model for nuclear scattering required multiple scattering events: hence the reason Rutherford used thin gold foil.