Neutrons: The Glue That Holds the Nucleus Together
It was a race against time. The Big Bang released protons, neutrons, electrons, and photons into the newly generated and rapidly expanding universe. With a half-life of only 10 minutes, though, the free neutrons rapidly plummeted toward extinction. What saved them? The protons! As detailed by Steven Weinberg in his excellent book, “The First Three Minutes,” the protons snapped up the neutrons within 10 minutes to form deuterium particles; neutrons combined with protons in this manner don’t decay. The deuterium particles then combined with each other to create the stable helium-4 nucleus, thus laying the groundwork for the later creation of the periodic table inside the stars as recounted here.
Why were the nearly-extinct neutrons “heroes” as opposed to simply survivors? Because of this: proton-proton nuclei aren’t stable. You can’t build the periodic table on protons alone.
Two forces are at play inside the nucleus: 1) electromagnetic repulsion between the positively charge protons, and 2) strong interaction attraction between all protons and neutrons. The proton-proton nucleus has too much of (1) and not enough of (2) to remain stable. The presence of neutrons increases (2) without affecting (1). And this is why the neutrons are the glue that holds the nucleus together and why the neutrons enabled creation of the periodic table.
In general, we don’t learn about this role of the neutron in school as center stage belongs primarily to the the chemistry of the electrons and then to the number of protons, which locates each element in its proper position in the periodic table. Yes, we bring in discussion of the neutron to explain the presence of isotopes but I’m not sure we spend time explaining or otherwise celebrating its glue-like tendency.
There’s another aspect of the neutron that supports the unsung hero claim. But first a short historical background to set the context.
The Discovery of the Neutron: A Turning Point in Physics
The year 1928 welcomed the arrival of quantum mechanics, which largely focused on the behavior of electrons about the nucleus. But did you know that all of this intense mathematical and experimental work was done prior to the discovery of the neutron?
You see, at this time physicists hypothesized that the nucleus contained only protons and electrons. For example, they assumed that the helium-4 nucleus consisted of four protons and two electrons, thus matching the atomic weight of 4 (electrons have negligibly low mass relative to protons) and the net atomic charge of +2 needed to neutralize the -2 charge of the orbital electrons. Since most experiments involving the disintegration of the nucleus resulted in the emission of either alpha particles (He-4 nucleus) or beta particles (electrons), there was no reason to think that any particles other than protons and electrons comprised the atom’s center or the atom itself, for that matter. It was in response to this situation that Otto Frisch wrote, “The complacency of 1930—the widespread belief that physics was nearly complete—was shattered by the discovery of subatomic particles.” [1]
Neutrons as Atomic Projectiles: A New Branch of Physics
In 1932 at the Cavendish laboratory, James Chadwick (1891-1974) discovered subatomic particles nearly identical to the weight of the proton and deduced that they must be electrically neutral on account of their showing high penetrating power into the electrically-charged environment around the nucleus. The realization of the neutron as the final significant elementary particle of the atom, until the discovery of the quark in the 1970s, quickly solidified the terminology of atoms and helped clarify the differences between atomic number, atomic weight, and isotopes.
The neutrality of the neutron subsequently inspired Enrico Fermi (1901-1954) to propose its use as an improvement over alpha particles as atomic projectiles. The positive charge of the alpha particles repels them from the positively charged nucleus; thus, only high-energy alpha particles stand a chance of penetrating the nucleus. The neutron, on the other hand, has no charge and can easily pass through the nucleus’ defenses without the need for high speed.
Latching onto Fermi’s idea, the team of Otto Hahn (1879-1968) and Lise Meitner (1878-1968) opened a new branch of physics when they started firing neutrons at different elements to study the radioactive disintegration process, uranium being a favorite target. Over the course of their program, they surprisingly found that the reactions seemed more probable the slower (less energetic) the neutron projectiles were, exactly opposite what they expected. At this time, most physicists felt that the disintegration resulting from such bombardments was caused by the physical smashing of one particle into another, much like a high speed bullet would shatter a rock. But Hahn and Meitner’s results suggested otherwise. It was the slow speed bullet that was the most effective. And it was the slow speed neutron that penetrated the uranium nucleus, caused it to split, and led to the discovery of fission. Another story for another time.
Thank you for reading my post about the neutron. I go into much greater detail about the atomic theory in Chapters 3 and 4 of my book Block by Block – The Historical and Theoretical Foundations of Thermodynamics.
Reference
[1] Frisch, Otto, The Nature of Matter, 1978, p. 428.
Identifying the Micro behind the Macro 