The Discovery of the Proton
Three Seminal Experiments in Early Atomic Physics
The 20th century saw a monumental change in our understanding of matter. New, exotic particles were discovered with dizzying frequency — the positron in 1932, the muon in 1937, the pion, kaon, and lambda in 1947, the neutrino in 1956, the first quarks in 1969, the vector bosons in 1983 — finally culminating in the discovery of the Higg’s boson, confirmed in 2013, another stunning success of the Standard Model. This remarkable progression is all the more incredible when we remember that at the turn of that very same century, we had yet to discover the humble proton.
The proton is somewhat unique in the history of particle discoveries because, unlike the many particles mentioned above, there does not exist a single moment that we can point to and say definitively, here we have our discovery. The history of the proton is inextricably bound to the whole history of atomic physics, stretching back to the first atomists of ancient Greece, and perhaps beyond. Even so, three experiments in particular were vital to our final, clear picture of the proton. The first was the famous gold foil experiment, which led to the discovery of the atomic nucleus. The second was an historic experiment by Rutherford, who became the first person to split the atom when he directed alpha rays through a gas, knocking (the yet-to-be-named) protons out of the nucleus. Finally, Aston’s experiments measuring atomic weights gave us the theory of isotopes, and put the proton in its proper place as a constituent of all elements.
Early Atomic Physics
Before we dive into this fascinating trio of experiments it’s important to understand some of the intellectual developments that preceded them. With that aim in mind, let’s take a quick tour through the proton’s history.
The first ideas about the proton can be traced back to our very first attempts at understanding matter. You can follow these theories back thousands of years, to the days when “the elements” meant earth, air, fire, and water.
Of course, our modern conception of the proton begins much later, with the English chemist William Prout (1785–1850). Prout was the first to notice that the atomic weights of the known elements were all integer multiples of the atomic weight of hydrogen. For example, a carbon atom is about twelve times as heavy as a hydrogen atom. The fact that all elemental weights were simple multiples like this led Prout to hypothesize in 1815 that the hydrogen atom was actually the most fundamental object. He named this object the “protyle”. Unfortunately Prout’s hypothesis was discarded some years later, when Jons Jakob Berzclius and Edward Turner carefully weighed various elements in 1828 and 1832 respectively, and found fractional weights. We’ll return to their finding later.
Atomic physics took another important step toward the proton in 1869, when Dmitri Mendeleev published the earliest version of our modern periodic table of the elements. Mendeleev’s main innovation was in ordering elements by their chemical properties, as opposed to the conventional classification by atomic weight. Mendeleev knew that the position of the atom in his table, i.e. its atomic number, had chemical significance, but had no explanation for the number itself. The mysterious atomic number begged for a physical explanation.
Fast forward another 28 years, to 1897. J.J. Thompson famously discovers the electron. This was the first time in human history that we found a particle that was not itself an atom, a huge leap forward for our understanding of matter, and another vital step on the journey to the proton’s discovery.
Two years later, in 1899, Ernest Rutherford discovered and named alpha and beta rays, with gamma rays following in 1903. The discovery of radiation led to a surge in new experiments. These new investigations came to a head in 1913 when Hans Geiger and Ernest Marsden fired alpha rays at gold foil to prove the existence of the atomic nucleus. They were knocking on the proton’s door.
The Gold Foil Experiment
The gold foil experiment is likely one of the most famous physical experiments in science, often taught to high school students or recreated in undergraduate physics labs. It is the first of the three seminal experiments that revealed the proton to the world, and we will explore it in detail now.
Although the gold foil experiments are often known by Rutherford’s name, the most significant study was actually performed by Hans Geiger and Ernest Marsden in 1913, and explained in their landmark paper entitled “The laws of deflexion of 𝛂 particles through large angles.”
The leading theory of atomic structure at the time was Thompson’s plum pudding model, which he proposed after discovering the electron. In this model, positive and negative charges were distrubuted evenly throughout an atom, like plums in a pudding. However, in the early 1910s, evidence had started to gather that contradicted this theory. If the theory were true, then charged beams should pass more-or-less straight through matter, with no deflection, since the equally spread out charges would not push the beam in any particular direction. Some early investigations using radiation showed that charged beams did not pass straight through matter, and these findings led Rutherford to postulate an alternative theory.
Rutherford’s nucleus theory, which he laid out in detail in 1911, posited that atoms did not have an equal distribution of charge, but instead had a small core of positive charge in their centers. He believed that this theory could explain some of the recent results, and produced detailed calculations that showed, among other things, exactly how beams should be deflected by atoms with a nucleus. Motivated by this experimental and theoretical progress, Geiger and Marsden designed their definitive experiment.
You can see what their setup looked like in this original drawing from their 1913 paper:
The experiment consists of a lead box (B) mounted on a rotatable platform (A), sitting in an airtight tube (C). The box contained a thin film of gold foil (F) and a radium source (R), which stayed fixed while the box was rotated. A diaphragm (D) ensured the particles emitted by the radium would be directed in a thin beam, aimed at the foil. The microscope (M) and scintillating screen (S) affixed to the end of the scope rotated with the box, so that measurements from the scintillator could be taken at different angles (from 5° to 150°) with respect to the beam direction. The measurements Geiger and Marsden recorded were simple counts, by eye, of the flashes of light that would occur on the zinc-sulphide screen each time it was struck with an alpha particle. Their goal was to discover the charge distribution within the atoms constituting the foil.
Their results included four key findings. First, different thicknesses of foil were used to determine the relationship between the number of flashes and the thickness of the target. Unsurprisingly, with a thicker foil target, fewer particles made it to the screen, fewer flashes. Second, by swapping out the gold for other materials, Geiger and Marsden observed a direct relationship between the number of flashes and the square of the atomic weight of the element used for the foil. This suggested that heavier elements had larger concentrations of charge inside of them. Third, they investigated the relationship of the number of flashes to the velocity of the particles, using mica sheets to slow them down, and their measurements matched Rutherford’s predictions. Their fourth and most important finding was the dependence of the number of flashes on the scattering angle. An equal number of flashes at different angles would suggest a uniform charge distribution in the material, like the plum pudding model predicted. Instead, the mathematical relationship they found exactly matched the formula predicted by Rutherford’s nucleus theory (see here for the detailed calculation). Each one of these four results was completely and accurately predicted by Rutherford’s model, and the nucleus theory was firmly established.
With the nuclues confirmed, it was time to find out what was inside. To do this we’ll return to the same alpha ray bombardment technique, this time masterfully employed by Rutherford himself. With these historic investigations Rutherford became the first person in history to split the atom.
Here’s what his setup looked like:
Just like in the gold foil experiment, the general idea behind the bombardment experiment was to shoot alpha particles at something, except now our target will be various gases instead of gold foil. To perform the experiment the chamber (C) was filled up with a particular gas, and the radium source (D) would then radiate the alpha rays through the gas. Rutherford could vary the distance between the radium source and the scintillator screen (S), by sliding the mount (B). As in the gold foil experiment, measurements were taken by observing the scintillations by eye, quite a tedious process. In an incredible first hand account, Rutherford describes the painstaking care with which he and his assistant, William Kay, made observations:
In these experiments, two workers are required, one to remove the source of radiation and to make experimental adjustments, and the other to do the counting. Before beginning to count, the observer rests his eyes for half an hour in a dark room and should not expose his eyes to any but a weak light during the whole time of counting. The experiments were made in a large darkened room with a small dark chamber attached to which the observer retired when it was necessary to turn on the light for experimental adjustments. It was found convenient in practice to count for 1 minute and then rest for an equal interval, the times and data being recorded by the assistant. As a rule, the eye becomes fatigued after an hour’s counting and the results become erratic and unreliable. It is not desirable to count for more than 1 hour per day, and preferably only a few times per week.
Let’s take a closer look at the fruits of their labor.
When the chamber was filled with gas, like oxygen or carbon dioxide, Kay and Rutherford saw the number of flashes drop. This result was expected, as the gas acts as a barrier, slowing the particles to a stop before they can reach the screen. However, when they let in air from the lab, they saw the number of flashes increase. Particles were seemingly produced from thin air. After carefully ruling out potential sources of contamination from water or dust, Rutherford hypothesized that it was actually the nitrogen in the air that was carrying these extra particles. When he introduced pure nitrogen into the chamber, the number of flashes rose even further, confirming his suspicions. Rutherford took many additional measurements and made many small variations to this experiment, for example by changing the pressure of the gas in the chamber, and testing the particle’s deflections in a magnetic field, which all further confirmed his theories.
The process taking place in the chamber was the chemical reaction ¹⁴N + 𝛂 → ¹⁷O + p (nitrogen plus alpha particles produces oxygen plus a proton). Rutherford had achieved nuclear transmutation. Although he didn’t know the exact details of the reaction taking place, the significance of his findings were by no means lost on him. Rutherford correctly concluded that bare hydrogen nuclei were being knocked out of the nucleus of nitrogen, and in the following year he dubbed these particles “protons,” in a nod to Prout.
Although at this point it is tempting to declare the proton found and be done with the matter, our picture of the proton was still incomplete. In the same year that Rutherford was making his observations with alpha particles, Francis Aston was conducting crucial research into atomic weight. Without Aston’s careful measurements, the proton could not have been properly understood as a primary constituent of all atomic nuclei, and so our “discovery” is not complete without a discussion of his ingenius invention, the mass spectrometer. This device, and his experiments with it, are the subject of our third and final experiment.
Aston’s original schematic is shown above. The most salient details include the discharge tube (B) where particles were ionized using X-rays, and then accelerated (to the left, in the diagram). The particles pass through an electric field (between J1 and J2), and a subsequent magnetic field (in the region M.) Aston’s insight was to carefully arrange these fields so that all particles having the same charge-to-mass ratio would end up at the same point on the detector plate (W). By studying one material at a time, the particles could be guaranteed to have the same electric charge, and so he could make relative measurements of the masses.
His results looked like this -
The mass is recorded in units relative to the atomic weight of oxygen which was taken to be exactly 16. Aston’s experiments led him to discover the isotopes. He found that although a random sample of an element wouldn’t have an integer multiple weight of hydrogen, the sample contained isotopes and these isotopes did have the whole number ratios of weights. These measurements explained Mendeleev’s atomic number, as well as Berzclius and Turner’s misleading results, and finally vindicated Prout, a century later. The protyle had been reborn as the proton, and thanks to these three experiments, it had been discovered, explained, and put it in its proper place as one of the core building blocks of all matter.
It is astounding to look back and realize that merely a century ago, when we had advanced so far as human beings — we’d invented radio, and the light bulb, we’d understood evolution and the germ theory of disease, Einstein had completed the theory of general relativity and already predicted the existence of gravitational waves — even after all of this incredible scientific progress, we still had no idea what made up the ordinary matter all around us.
The discovery of the proton permanently changed our understanding of matter, and ushered in a new era of science. Unlike many later particle discoveries, the proton was not found by any single experiment, and it was found despite the fact that no one was looking for it. The three experiments we explored above are perhaps the most directly significant with respect to the proton’s discovery, but there was a great deal of complementary research that was also essential. Many experiments, and many scientists, including Thomson, Rutherford, Chadwick (discoverer of the nuetron), Geiger, Marsden, Darwin, Aston, Curie, and others, were integral to our understanding of this basic building block of matter. Rutherford was especially prolific, pioneering an entirely new field of experimental physics. Today our understanding of the proton has evolved thanks to quantum mechanics, but its history and the techniques used to investigate it will never lose their significance.