The question of who first split the atom marks a profound turning point in human understanding of matter and energy. This event refers specifically to induced nuclear fission: the violent division of a heavy atomic nucleus into two or more smaller fragments. This reaction releases an immense amount of energy. The discovery forever altered the landscape of physics and introduced the world to the nuclear age.
Early Attempts at Atomic Transmutation
Long before true nuclear fission, scientists manipulated the atomic nucleus in attempts to change one element into another, a process known as transmutation. Ernest Rutherford first demonstrated an artificial nuclear reaction in 1919 by bombarding nitrogen gas with high-energy alpha particles. This collision caused the nitrogen nucleus to emit a proton, transforming it into an oxygen isotope.
This foundational work showed that the nucleus could be altered by external particle bombardment. In the 1930s, the focus shifted with the discovery of the neutron, a particle with no electrical charge. Enrico Fermi began using these neutral particles to bombard heavy elements like uranium, theorizing that the neutron could easily penetrate the positively charged nucleus. Fermi’s experiments led him to believe he had created new “transuranic” elements, but he had unknowingly been performing the very first fission events.
The Experimental Discovery of Fission
The actual physical splitting of the uranium nucleus was definitively accomplished by chemists Otto Hahn and Fritz Strassmann in Berlin in late 1938. They bombarded uranium with slow-moving neutrons and carefully analyzed the resulting radioactive decay products. Their expectation, following prevailing scientific theory, was to find elements slightly lighter or heavier than uranium.
Instead, their chemical analysis yielded an unexpected result: the presence of Barium in the residue. Barium (Z=56) has an atomic number roughly half that of Uranium (Z=92). This difference was far too large to be explained by any known nuclear decay process. Hahn and Strassmann were surprised by this finding, stating they were “obliged” as chemists to confirm the barium but could not yet accept the physical implications. Their experimental proof that the uranium nucleus had broken into smaller fragments was the definitive moment of discovery.
The Physics of Nuclear Splitting
The mechanism for nuclear splitting begins with a slow-moving neutron striking a heavy nucleus, specifically the rare uranium isotope, Uranium-235. The U-235 nucleus absorbs the neutron, immediately becoming the unstable compound nucleus Uranium-236. This absorption introduces enough excitation energy to destabilize the massive nucleus.
Physicists visualize this process using the liquid drop model, which treats the nucleus like a droplet of charged liquid. The incoming neutron causes the “drop” to oscillate and deform into an elongated, dumbbell shape. The short-range strong nuclear force holding the nucleons together is overcome by the long-range electrical repulsion between the protons. At a critical point, the repulsive force causes the nucleus to pinch off and split. This results in two smaller, highly energetic nuclei, known as fission fragments, and releases a tremendous amount of binding energy.
Interpreting the Experimental Results
The chemical evidence from Hahn and Strassmann required a theoretical explanation to confirm the physical event. This crucial step was provided by physicist Lise Meitner and her nephew, Otto Frisch, who had fled Nazi Germany. Meitner received a letter describing the presence of Barium and immediately recognized its significance.
Meitner and Frisch used Niels Bohr’s liquid drop model to calculate the energetics of the process. Meitner determined that the splitting of the uranium nucleus would release approximately 200 megaelectron volts (MeV) of energy, confirming a violent break. Frisch coined the term “fission” for this process, borrowing the word from the biological term for cell division. This theoretical paper, published in February 1939, provided the definitive physics framework for the experimental discovery.
The Concept of a Nuclear Chain Reaction
The most significant consequence of the fission discovery was the realization that the splitting nucleus released additional free neutrons, typically two or three per split. This release of secondary neutrons transformed nuclear fission from a scientific curiosity into a potential global force.
The extra neutrons meant that each single fission event could trigger the fission of other nearby uranium nuclei. This is the mechanism for a self-sustaining nuclear chain reaction, where the process accelerates exponentially. If enough fissile material is present, known as reaching critical mass, the reaction can rapidly multiply, releasing energy on a massive scale. This cascading reaction immediately signaled the possibility of both a powerful new energy source and a devastating military weapon.