The discovery of radioactive decay fundamentally changed the understanding of matter, revealing that the atom was not an indivisible, unchanging particle. This process occurs when an unstable atomic nucleus spontaneously loses energy by emitting radiation. The finding demonstrated that elements could transform, challenging the principles of classical physics and chemistry. It initiated the field of nuclear science, leading to advancements in medicine, energy, and our view of the cosmos.
The Initial Observation
The first evidence of this phenomenon came from the French physicist Henri Becquerel in 1896, following the excitement over the recent discovery of X-rays. Becquerel was investigating phosphorescence, the property of certain materials to glow after exposure to light, and was trying to see if these materials also emitted X-ray-like radiation. On February 26, 1896, he prepared an experiment by placing uranium salts on a photographic plate wrapped in thick black paper, intending to expose the setup to sunlight.
Due to overcast weather in Paris, he placed the materials in a drawer for several days, delaying the experiment. When he later developed the photographic plate, he found to his surprise that the uranium salts had left a clear, dark impression, just as if the plate had been exposed to light. This fogging indicated that a penetrating, invisible radiation was being emitted spontaneously by the uranium, without any need for external energy or sunlight to excite the material.
Becquerel’s subsequent experiments confirmed that this penetrating radiation was an inherent property of the uranium element itself, regardless of the chemical compound or physical state of the salts. He had accidentally discovered a completely new physical process—the spontaneous emission of energy, which was initially called “Becquerel rays.” This observation established that certain elements possess an internal energy source allowing them to continuously emit radiation.
Defining the Phenomenon
Building on Becquerel’s initial discovery, the Polish-French physicist Marie Curie chose to study these mysterious rays for her doctoral thesis, using a sensitive electrometer developed by her husband, Pierre Curie. Marie’s systematic testing of various elements revealed that thorium also emitted the same kind of radiation. She made the realization that the intensity of the radiation depended only on the amount of the radioactive element present, not on its chemical form, concluding that the radiation was an intrinsic property of the atom itself.
The Curies then examined uranium ores, such as pitchblende and chalcolite, and found that some samples were significantly more radioactive than pure uranium. This led them to hypothesize that the ores contained minute quantities of one or more undiscovered elements that were far more radioactive than uranium. They began the arduous process of chemically separating the components of pitchblende, using radiation measurements to trace the highly active fractions.
This exhaustive work led to the discovery of two entirely new elements. In July 1898, they announced polonium, named after Marie’s native Poland, and five months later, they announced radium, named from the Latin word for ray. To describe this new property of matter, Marie Curie coined the term “radioactivity,” defining the process as the spontaneous emission of radiation by certain elements.
Understanding the Mechanism of Nuclear Change
While the Curies defined the phenomenon, the mechanism of the process—the actual change occurring within the atom—was elucidated by later scientists, most notably Ernest Rutherford. Rutherford, working with others, demonstrated that radioactivity involved a spontaneous transformation of an unstable atomic nucleus. This transformation releases energy and particles, often changing the identity of the parent atom into a different element.
Rutherford identified and named the three primary types of emissions based on their penetrating power. Alpha decay involves the emission of an alpha particle, which is essentially the nucleus of a helium atom, consisting of two protons and two neutrons. This emission reduces the atomic number of the parent nucleus by two and its mass number by four, thereby creating a new element.
Beta decay occurs when a neutron inside the nucleus converts into a proton, resulting in the emission of a high-energy electron, known as a beta particle. This conversion increases the atomic number by one while the mass number remains nearly the same, resulting in a nuclear transmutation.
The third type is gamma decay, which is the release of excess energy from an excited nucleus in the form of a high-energy electromagnetic wave called a gamma ray. Gamma decay typically follows alpha or beta decay, bringing the nucleus to a more stable energy state without changing the element’s identity.