The discovery of how to split the atom represents a profound turning point in human history and scientific understanding. This breakthrough fundamentally altered our perception of matter and energy, revealing a powerful source previously locked within the atomic nucleus. This moment ushered in a new era of scientific endeavor and technological capability.
Defining Atomic Splitting
The term “splitting the atom” specifically refers to nuclear fission, where the nucleus of a heavy element is divided into two or more smaller, lighter nuclei. When a heavy nucleus like uranium undergoes fission, the combined mass of the resulting fragments is slightly less than the original nucleus. This small difference in mass is converted directly into kinetic energy, following Albert Einstein’s mass-energy equivalence principle, \(E=mc^2\). A single uranium nucleus undergoing fission releases approximately 200 megaelectronvolts of energy, a large amount compared to the energy released in chemical reactions.
The Precursors to Fission
The path to discovering fission relied on decades of prior work in nuclear physics and radioactivity. For example, in 1917, Ernest Rutherford demonstrated the first artificial transmutation by bombarding nitrogen with alpha particles. This experiment changed one element into another, but it only knocked a small particle off the nucleus, unlike the massive breakage characteristic of fission.
In the mid-1930s, the Italian physicist Enrico Fermi conducted crucial experiments by bombarding uranium with newly discovered neutrons. He observed new radioactive products and believed he had created elements heavier than uranium, known as transuranic elements. Although Fermi had unknowingly caused fission, he misinterpreted the results due to the complexity of the chemical analysis, narrowly missing the correct conclusion that lighter elements had been produced.
The Moment of Discovery
The experimental breakthrough occurred in Berlin in late 1938 at the Kaiser Wilhelm Institute for Chemistry. Chemists Otto Hahn and Fritz Strassmann were systematically bombarding uranium with slow neutrons in an effort to characterize the supposed transuranic elements. Their meticulous chemical analysis of the reaction products yielded a baffling result that contradicted all prevailing nuclear physics theories.
The pair repeatedly identified barium, an element significantly lighter than uranium, among the reaction products. This meant the uranium nucleus must have been nearly halved in size. Hahn wrote to his former colleague, Lise Meitner, expressing his astonishment and uncertainty about the physical mechanism behind their conclusion. Hahn and Strassmann had experimentally achieved the complete splitting of the uranium nucleus, but they could not provide a physical explanation for the unprecedented phenomenon.
The Confirmation and Implications
The theoretical explanation for the experimental results came from physicist Lise Meitner and her nephew, Otto Frisch. Meitner, a Jewish Austrian scientist, had fled Nazi Germany months earlier and was in exile in Sweden when she received Hahn’s letter detailing the baffling barium result. While walking in the snow, Meitner and Frisch applied a model that treated the nucleus like a liquid drop.
They reasoned that the uranium nucleus, when struck by a neutron, became unstable, elongated, and split into two smaller “drops”. Meitner calculated that the kinetic energy of the resulting fragments precisely matched the mass loss predicted by Einstein’s equation. Frisch coined the term “fission” for this new nuclear process, recognizing its similarity to the division of biological cells. The two quickly published their theoretical findings, confirming that the atom had been split. Crucially, the discovery included the realization that fission released additional neutrons, creating the potential for a self-sustaining nuclear chain reaction with profound implications for energy and military technology.