By Don Lincoln, Fermilab
J. J. Thomson’s model of how atoms made molecules involved little spheres that touched one another and held together, like a bunch of sticky marbles or something like that. However, we know that his idea was wrong. How did that come to be? It actually was his student, New Zealand physicist Ernest Rutherford who figured it out in 1911, when he discovered the nuclear center of an atom.
Rutherford Discovering Proton
By 1918, scientists had begun to isolate the nuclei of atoms and the smallest nuclei they had found was the hydrogen nucleus. Hydrogen nuclei were observed by shooting alpha particles at a tube full of hydrogen and occasionally the alpha particle would knock a hydrogen nucleus out of the atom. Those nuclei hit detectors that gave of twinkles of light when they were hit.
The way that Rutherford discovered the proton was in the following indirect way. He was shooting alpha particles through air, which is mostly nitrogen, with some oxygen and traces of other gasses. When he did this, he occasionally would see what looked like twinkles that occurred when the nuclei of hydrogen atoms hit his detector. The twinkles of the light were very clear.
Now, it could have been that what he was seeing was the alpha particles hitting hydrogen in the air, or maybe water molecules, which we know are made of two hydrogen atoms and one oxygen. So, he had to figure that all out.
He did so by repeating the experiment, this time with vials of pure nitrogen gas, pure oxygen gas, etc. What he found out was that it was the nitrogen that was the source of the hydrogen nuclei signal. He didn’t see it very much at all with oxygen.
Findings from the Experiment
So what he had done was use alpha particles to knock protons out of nitrogen nuclei, but he didn’t know that of course. All he knew was that he could make hydrogen nuclei by hitting nitrogen nuclei and, of course, that’s a bit puzzling. It took him a long time to figure it out.
Depending on how one reads the history in detail, it took between a year and six years for him to work through his thoughts. He saw the first creation of hydrogen nuclei in 1918 and it took until 1919 or perhaps until 1925 before he and others had put the entire picture together.
They realized that nitrogen nuclei must contain hydrogen nuclei and that they could be knocked out. Rutherford then coined the term proton, from the Greek word protos, which means first. The number of known constituents of the atom was now two. But there was a problem.
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Problem of the ‘Missing’ Mass
Scientists had figured out the mass of the proton and the electron and they knew from the fact that the atom was electrically neutral that they had to come in equal quantities. The problem was that the masses didn’t add up.
There were atoms with a known number of electrons, and therefore protons, and the mass of the atom was too big. There had to be another source of mass in the atoms and that source had to be electrically neutral. This is what we call the neutron these days, but they didn’t know that back then.
Now with the discovery of the neutron, Rutherford wasn’t the hero; it was his student, British physicist James Chadwick. In fact, he was the last of a scientific dynasty, with Thomson who discovered the electron, followed by his student Rutherford, who discovered the atomic nucleus and the proton, and finally Chadwick, his student, and discoverer of the neutron.
At any rate, Chadwick discovered the neutron by using a radioactive element called polonium to shoot alpha particles at beryllium, which is an element for which it is easy to knock neutrons out, although he didn’t know that then. What he knew is that smacking alpha particles into beryllium kicked out electrically-neutral radiation, which we now know as neutrons.
Discovery of Neutron
Chadwick then took those neutrons and ran them into a block of paraffin, which is basically wax and is relevant because it is chock-a-block full of hydrogen. Those neutrons crashed into the protons and knocked out the protons, which were detected much like Rutherford did.
In fact, that’s how the neutron was discovered. Since neutrons are neutral, the technology of the day didn’t see them when they got knocked out of the beryllium. Chadwick noticed that protons got knocked out of the wax when it was brought near the polonium and he realized that they were getting knocked out by electrically neutral radiation.
It wasn’t until 1932 that Chadwick discovered the neutron, which was 14 years after Rutherford had seen the first signs of the proton.
Completing the Basic Structure of an Atom
With the discovery of the neutron, our understanding of basic structure of the atom was complete. Protons and neutrons are held together tightly in a small ball at the center of the atom called the nucleus. Surrounding the nucleus is a much larger volume—a trillion times larger in fact—where the electrons can be found.
With that model, scientists can work out the details of chemistry and nuclear physics. This very impressive achievement was all wrapped up by the early 1930s. Surely, scientists have made advances since then.
Common Questions about the Discovery of Proton and Neutron
The New Zealand physicist Ernest Rutherford discovered nuclear center of an atom in 1911.
Ernest Rutherford coined the term proton, from the Greek word protos, which means first.
It was in1932 that James Chadwick discovered the neutron, which was 14 years after Ernest Rutherford had seen the first signs of the proton.