You probably don’t remember when you learned that the atom has a nucleus. You saw so many depictions of the atom with its compact nucleus that it seems like nothing special. It’s kid stuff, Right? They teach about it in elementary school. How hard could it be?
But the discovery of the nucleus was unexpected, and recent. It was only roughly 106 years ago. Knowledge of the nucleus was younger than three of my grandparents. I didn’t think of that when I first learned about it when I was a kid. I wish someone had explained that to me. I would have appreciated the perspective. I figured if they taught it to kids it couldn’t be that advanced, and it surely must be older than all that stuff Einstein did, whatever that was. But the nucleus is newer than most of what Einstein is famous for. Kid stuff is newer than relativity.
Knowledge of the nucleus dates back to a series of experiments conducted between 1909 and 1911 suggested and supervised by Ernest Rutherford. This was the foundational experiment in nuclear physics. Rutherford already had a Nobel Prize at that point for building on the work of Marie Curie and Henri Becquerel by making further distinctions between different types of radiation. Wikipedia and physics textbooks usually say he established that the alpha particle, which he named, was the same as the helium nucleus. Well, he kind of did this, but he did not know at the time that helium or an atom of any other element even had a nucleus. No one knew that. What he established was that there was a distinct class of particles that were very dense and strongly charged and that they shared some characteristics with ionized helium gas. And remember, no one at that time knew as much about ions as a typical high school chemistry student does today.
Physicists in the early twentieth century were pretty sure that electrons were real. Rutherford’s advisor at Cambridge, J.J. Thompson had established that cathode rays had a fixed charge to mass ratio, and later experiments showed that there seemed to be a lower limit to the amount of negative electrical charge that could be transferred in one event. This strongly suggested that there was some discrete bit of matter, a quantum, that carried the smallest negative charge possible, a particle that was named the electron.
Thompson proposed a model of the atom based on this discovery, the “plum pudding” model, a ball of positively charged matter containing electron plums. This model was met with only a few objections mostly because no one had any better ideas. At least not until Rutherford and two of his students at Manchester, Hans Geiger and Ernest Marsden, decided it might be interesting to “probe the pudding” with alpha particles to see if they could find out anything about the internal structure of the atom. They decided to use gold foil for their experiment because gold is just about the most malleable metal, and extremely thin and uniform sheets were commercially available. Thinness and high density were important because the experiment required that each alpha particle interact with as few atoms as possible.
The expected result was that there would be almost no interaction between the beam and the foil. I know that sounds implausible because I’ve said that alpha particles are almost entirely stopped by your outer layer of dead skin cells. You must remember that gold can be pressed into foils that are so thin that the lifeless proteins of your outer skin are massively thick in comparison. The expected result for gold foil in the plum pudding model was that the particle beam would become slightly unfocused.
Records from the time are unclear why, but Rutherford suggested that Geiger and Marsden also check for unexpected results. This may have been due to intuitions about the atom or simply Rutherford’s usual scientific rigor. I suspect simply the latter because until the end of his life in 1936 Rutherford never indicated the results of this experiment were anything other than a complete surprise. And the results were quite shocking. Almost all of the alpha particles went straight through the foil as if it did not exist. Some particles were deflected slightly as would be consistent with the plum pudding model. But others were deflected at very large angles, even close to 90 degrees. Even more shocking was that a small number of particles bounced out of the foil back toward the radiation source.
I have had the good fortune of taking a class where we duplicated this experiment and played around with the difficult math of trying to figure out what was going on. I had read about this experiment a few times both on my own as a kid and in school. I never really felt how cool it was until I got a chance to play with radioactive materials and screw up the math.
I really think this is best explained with a video. There are almost too many videos on this subject on YouTube, and most are misleading. The test apparatus has to be small, and the radiation detector has to close to the gold foil. Too many air gaps will make the air have many times the effect of the gold foil and ruin the results.
This video with a real apparatus replicating the experiment with more modern equipment captures the scale more than any of the animated videos on YouTube:
This is the closest I could find to how it was actually done and is similar to the apparatus I used when I had the chance to duplicate this experiment. You could do this experiment on your kitchen table with the Americium capsule from a smoke detector, a tunable commercial radiation detector, and gold foil from an art supply store. You’d need a tunable detector because modern detectors are generally too sensitive, and you’d need to be able to screen out beta radiation coming from your own body to reliably measure the less common deflections from the experiment.
It’s important to note that Geiger and Marsden did not have this kind of radiation detection equipment. They used a small sheet of cloth permeated with zinc sulfide which fluoresces when struck by charged particles. This was first developed by Becquerel and also used by the Curies. The fluorescent screen was attached to a low power microscope to magnify rare events. While putting your eye near such a radiation source seems like a bad idea, optical glass is impervious to alpha particles, and even the closed eyelid of the other eye is plenty of protection.
After over a year of such laborious observations Geiger really wanted to find a better way to measure radioactive events and would invent radiation detectors that would simplify things. These became popularly known as Geiger counters. Radiation events cause a voltage change in a tube to create an electrical signal. This signal can be amplified and connected to a speaker to produce the ticks and pops associated with radiation detectors in popular culture, or connected to an electromechanical counter.
When Rutherford and his team published their results in 1911, they did not quite have the modern idea of the nucleus. While they favored the idea of a massively dense positive nucleus, they could not rule out the idea of a compact electron swirl zone in the center of a positively charged pudding. Hyperbolic deflection off of a massive positive core seemed simpler, but elliptical looping around an electron swirl could not be ruled out. All evidence seemed to indicate that electrons should be on the outside, and independent research would firmly establish that the nucleus was intensely heavy and positive within a year. Either way, Rutherford’s team was able to establish that the nucleus, whatever it was, was at least 12,000 times smaller than the outer chemically active zone of the atom.
Turns out, his numbers are right, and Rutherford’s hunch that the electrons are on the outside was right. This is why you’ve heard that the atom is mostly empty space. But Rutherford’s nucleus would birth quantum physics through his student, Nils Bohr. I think the idea that atoms have empty space is misleading. The macro scale idea humans have of edges and distinct objects sort of breaks down at the atomic level.
The nucleus is not kid stuff, though a child can sort of understand it, The nucleus is newer than powered flight, and newer than Einstein’s most famous equation. And progress was fast. It was just a little more than thirty years from Rutherford’s published results to the first atomic bomb.
How Einstein’s most famous equation became a pop culture meme in the fifties and sixties even though it really had not that much to do with nuclear physics. Who made the connection between Einstein and Kernspaltung? That was pretty awesome even if in practice Kernspaltung didn’t really require her analysis.
It’s Lise Meitner week next week unless newly confirmed Energy Secretary Rick Perry moves me to Oak Ridge. And if Putin reveals a third not quite new missile I’ll write about that instead.
I so wish I had been at Perry’s confirmation. I would have asked him whether gold foil meant anything to him.