Otto Hahn and Fritz Strassmann, two chemists working in Berlin, were the first to demonstrate that a uranium atom could be split into lighter elements, publishing their findings in January 1939 after a crucial experiment on December 17, 1938. But pinning the discovery of nuclear fission on two names flattens a story that stretches across decades and involves physicists and chemists on multiple continents, some of whom came agonizingly close to the answer years earlier and at least one of whom was deliberately written out of the credit she deserved.
Rutherford and the First “Splitting” of an Atom
The phrase “splitting the atom” entered popular vocabulary long before anyone achieved fission. In 1917, Ernest Rutherford, working at the University of Manchester, fired alpha particles (helium nuclei) at nitrogen gas and noticed that the collision knocked loose hydrogen nuclei, which he called protons. Published in 1919, the result showed that one element could be transformed into another: nitrogen was converted into an isotope of oxygen. Rutherford had achieved the first artificial transmutation, and newspapers quickly dubbed it “splitting the atom.”
Rutherford’s achievement was genuinely groundbreaking, but it was not fission. He was chipping a small piece off one nucleus by hurling another nucleus at it. Nuclear fission, as the term came to be defined, involves a heavy nucleus breaking apart into two roughly equal fragments, releasing a vastly larger amount of energy. The distinction matters because the energy released in Rutherford’s transmutation was tiny compared with what fission would later unleash. Rutherford himself famously dismissed the idea that atomic energy could ever be harnessed on a practical scale, calling it “moonshine.” He would be proven spectacularly wrong within two decades of his death.
Fermi’s Neutron Experiments and the Wrong Conclusion
The discovery of the neutron by James Chadwick in 1932 changed the game. Because neutrons carry no electric charge, they can slip past the electrical barrier surrounding a nucleus far more easily than alpha particles can. Enrico Fermi, working in Rome, seized on this. Beginning in 1934, his group systematically bombarded elements across the periodic table with neutrons, cataloging the radioactive products that resulted. When they hit uranium, the heaviest naturally occurring element, they detected unfamiliar radioactive substances. Fermi’s team concluded they had created new “transuranic” elements, elements heavier than uranium that did not exist in nature.
This interpretation was celebrated. Fermi received the Nobel Prize in Physics in 1938, and the transuranic interpretation was widely accepted. But it was wrong. The radioactive substances Fermi’s team had detected were not superheavy new elements. They were fragments of uranium atoms that had been split in two. Fermi had unknowingly achieved fission four years before anyone recognized it.
The Chemist Who Saw It Coming
One person did recognize the possibility almost immediately. Ida Noddack, a German chemist who had co-discovered the element rhenium, published a paper in 1934 arguing that Fermi’s team had not adequately ruled out a simpler explanation. She suggested that the uranium nucleus, when hit by a neutron, might break apart into several large fragments rather than just gaining a particle and becoming something heavier. She anticipated the possibility of nuclear fission a full nine years before it was experimentally confirmed.1Notes and Records: the Royal Society Journal of the History of Science. A tale of oblivion: Ida Noddack and the ‘universal abundance’ of matter
Noddack’s suggestion was ignored. Part of the reason was scientific: no theoretical framework existed at the time to explain how a nucleus could split into large pieces. The prevailing model treated the nucleus as a tightly bound system; breaking it in half seemed to require implausibly large energies. Part of the reason was social. Noddack was not part of the inner circle of nuclear physicists, and her earlier claim to have discovered element 43 (which she called masurium) had been disputed and never confirmed by others, which damaged her credibility. She did not follow up with experiments of her own, and the idea lay dormant. Had anyone taken her seriously enough to test the hypothesis, fission might have been discovered years earlier.
December 1938 in Berlin
The actual discovery came from an unlikely combination of chemistry and persistence. Otto Hahn and Fritz Strassmann, working at the Kaiser Wilhelm Institute for Chemistry in Berlin, had been repeating and refining the neutron-bombardment experiments on uranium for several years. Like Fermi, they initially believed they were producing transuranic elements. But their meticulous chemical analyses kept producing results that did not fit the transuranic model. Certain radioactive products behaved chemically like radium, which was confusing enough. Then, on December 17, 1938, after rigorous separation tests, they found that the substance they had identified as radium was actually barium, an element with roughly half the atomic weight of uranium.2Angewandte Chemie International Edition in English. Five Decades Ago: From the “Transuranics” to Nuclear Fission
Hahn was stunned by his own results. Barium has an atomic number of 56; uranium’s is 92. There was no known process by which bombardment with a single neutron could produce barium from uranium. The finding implied that the uranium nucleus had broken into two large pieces, something that had never been observed and that contradicted the reigning assumptions about nuclear stability. Hahn and Strassmann published their chemical evidence in the journal Naturwissenschaften in January 1939, cautiously reporting the result and acknowledging they could not explain it physically.
Meitner and Frisch Provide the Explanation
The physical explanation came almost immediately, from two people who had every reason to be involved in the Berlin experiments but were not. Lise Meitner, an Austrian-born physicist, had been Hahn’s closest collaborator for thirty years. She had co-led the uranium research program and was deeply involved in designing the experiments that led to the barium finding. But Meitner was Jewish, and after Germany annexed Austria in March 1938, she lost her Austrian citizenship and the protection it had provided. She fled to Sweden in July 1938, just months before the crucial experiment.
Hahn wrote to Meitner about the barium result as soon as he had it, asking for a physical interpretation. Meitner discussed the problem with her nephew, Otto Robert Frisch, a physicist working in Copenhagen under Niels Bohr. During a now-famous conversation over Christmas 1938, the two worked out the essential physics. Using the liquid-drop model of the nucleus, which treats the nucleus as a droplet of incompressible fluid held together by surface tension, they realized that a neutron could destabilize a heavy nucleus enough for it to elongate and pinch apart into two roughly equal fragments. The electrical repulsion between the protons in each half would then drive the fragments apart with enormous energy.
Meitner and Frisch calculated that the energy released per fission event was roughly 200 million electron volts, a figure consistent with the mass difference predicted by Einstein’s mass-energy equivalence. Frisch coined the term “fission,” borrowing it from biology, where it describes cell division. Their paper, published in Nature in February 1939, gave the world not just a name but a framework for understanding what Hahn and Strassmann had found in the lab.
The question of credit has never been fully settled. Hahn received the Nobel Prize in Chemistry in 1944 for the discovery of fission. Meitner was not included, despite her decades of collaboration and her critical role in the theoretical interpretation. Many historians and scientists regard this as one of the most significant omissions in Nobel history. Strassmann, too, was excluded from the prize, though he shared subsequent recognition with both Hahn and Meitner in later years.
Why Fission Releases So Much Energy
Fission stood out from every nuclear reaction previously observed because of its sheer scale. The fragments produced carry more than ten times the total ionization of anything known from earlier nuclear experiments.3Physics Bulletin. The discovery of fission The reason comes down to the balance between the strong nuclear force, which holds protons and neutrons together, and the electromagnetic repulsion between protons, which tries to push them apart. In very heavy nuclei like uranium, the repulsive force nearly matches the attractive one. The addition of a single neutron can tip this balance, causing the nucleus to deform and split. When it does, the two fragment nuclei snap into more stable configurations, and the excess energy that had been locked up holding the oversized original nucleus together is released as kinetic energy of the fragments, gamma radiation, and free neutrons.
The key detail, worked out by Niels Bohr and John Wheeler in their landmark 1939 paper, was that not all uranium atoms respond the same way to neutrons. Uranium-235, the rarer isotope making up less than one percent of natural uranium, undergoes fission when struck by slow-moving neutrons. Uranium-238, which makes up the overwhelming majority, requires much faster neutrons and is far less likely to fission under ordinary conditions.4Physical Review. The Mechanism of Nuclear Fission This distinction would prove pivotal: it meant that to build either a reactor or a weapon, you needed to either enrich the proportion of uranium-235 or find another fissile material entirely, a challenge that shaped the entire trajectory of nuclear technology.
The Chain Reaction and the Race to Weaponize
Fission by itself is a laboratory curiosity. What made it terrifying, and eventually transformative, was the realization that each fission event releases additional neutrons, which can trigger further fissions, which release still more neutrons, and so on. If enough fissile material is gathered in one place, this chain reaction can sustain itself or even accelerate explosively.
The chain-reaction possibility was demonstrated experimentally by Frédéric Joliot, Hans von Halban, and Lew Kowarski in Paris in early 1939. Their work confirmed that uranium fission released enough secondary neutrons to make a self-sustaining reaction plausible.5Comptes Rendus. Physique. The discovery of the neutron and its consequences (1930–1940) This finding transformed nuclear physics from a purely academic pursuit into a military science practically overnight. The timing was devastating: World War II began in September 1939, and governments on both sides quickly grasped the implications.
Leó Szilárd, a Hungarian-born physicist who had independently conceived of the chain-reaction idea even before fission was discovered, was among the first to push for secrecy. He persuaded Fermi and others to stop publishing their results openly and, together with Eugene Wigner and Edward Teller, drafted the famous letter to President Roosevelt, signed by Albert Einstein, warning that Germany might develop an atomic bomb. This letter led directly to the creation of the Manhattan Project. By December 1942, Fermi’s team in Chicago achieved the first self-sustaining nuclear chain reaction in a makeshift pile of uranium and graphite blocks beneath the bleachers of a university football stadium. Less than three years later, the bombs dropped on Hiroshima and Nagasaki ended the war and opened the nuclear age.
Why the Credit Question Is Still Debated
Assigning credit for fission’s discovery is complicated because chemistry and physics made different contributions at different moments, and the political context stripped some contributors of the recognition they earned. Hahn and Strassmann produced the irreplaceable chemical evidence: without their analytical rigor in identifying barium, the discovery would not have happened when it did. But Meitner’s role was not simply interpretive. She had been part of the research program from its inception, helped design the experimental approach, and her correspondence with Hahn during the critical weeks directly shaped his understanding of what the results meant. Frisch, working with Meitner, designed the first physical experiment to confirm fission independently, detecting the energetic fragments with an ionization chamber in Copenhagen just days after their theoretical paper.
Noddack’s earlier hypothesis adds another wrinkle. She proposed the right idea in 1934 but could not back it up experimentally, and the physics community largely dismissed her. Some historians argue she deserves more credit than she typically receives; others counter that a speculation without follow-through does not constitute a discovery. The debate reflects a genuine tension in how science assigns credit: is it the idea, the experiment, or the explanation that counts? In the case of fission, all three were performed by different people.
The Nobel committee’s decision to award Hahn alone remains controversial. Sweden’s own archives, opened decades later, reveal that the committee seriously considered Meitner but ultimately decided the prize should go to the chemical discovery rather than the physical interpretation. Strassmann, who had stood by Meitner personally during the Nazi period at considerable risk to himself, was also left out. The resulting narrative, in which a single male scientist is credited with a fundamentally collaborative discovery, has become a case study in how institutional biases shape the history of science.
Fission Before Humans
In a twist that reframes the entire human story, nuclear fission occurred naturally on Earth long before any scientist conceived of it. In 1972, French scientists analyzing uranium ore from the Oklo mine in Gabon, West Africa, discovered that the isotopic ratios were off. The uranium-235 concentration was significantly lower than expected, as though some of it had already been consumed in fission reactions. Further investigation revealed that roughly two billion years ago, when Earth’s atmosphere had become oxygen-rich enough to dissolve and concentrate uranium in groundwater deposits, and when the natural proportion of uranium-235 was above three percent (high enough to sustain a chain reaction with water as a moderator), spontaneous nuclear reactors had operated underground for hundreds of thousands of years.6Comptes Rendus. Physique. 2 billion year old natural analogs for nuclear waste disposal: the natural nuclear fission reactors in Gabon (Africa)
These natural reactors were not explosive. They were moderated by groundwater, which slowed neutrons to the speeds needed to split uranium-235, and they likely operated in cycles: the heat from fission would boil away the water, shutting the reaction down until fresh water seeped back in. The Oklo reactors ran at modest power levels, estimated at around 100 kilowatts, roughly the output of a small generator. But they ran for an extraordinarily long time, producing the same fission products and transuranics that modern reactors generate. The geological containment of those waste products over two billion years has made Oklo a valuable natural laboratory for studying long-term nuclear waste storage, offering evidence about how radioactive materials migrate through rock over timescales that no human experiment could replicate.