The scientific partnership of Marie and Pierre Curie fundamentally changed the understanding of matter through their investigation of rays emitted by uranium. Marie Curie discovered that the uranium ore pitchblende emitted rays far more intensely than pure uranium or thorium. This discrepancy led her to hypothesize that the ore contained unknown substances that were significantly more potent sources of this energy. Pierre Curie joined her research, leading to the identification of two previously unknown radioactive elements within the ore.
Polonium The First New Element Identified
The first new element was identified in July 1898, after the Curies chemically separated a fraction of pitchblende that co-precipitated with bismuth. This fraction was about 300 times more intensely radioactive than pure uranium, and its intense activity served as the primary evidence for its existence. Because of its close chemical behavior, the unknown element was difficult to separate from bismuth. They named this new element polonium (Po), in honor of Marie Skłodowska-Curie’s native country of Poland.
Radium The Element Requiring Massive Effort
Following the discovery of polonium, the Curies focused on a highly active fraction of pitchblende containing barium. They announced the existence of a second element in December 1898, naming it radium (Ra), derived from the Latin word for ray. Radium proved to be an order of magnitude more potent than polonium, with its compounds up to 900 times more radioactive than uranium.
The difficulty in isolating radium stemmed from its minute concentration within the raw ore. To obtain enough material to prove its atomic weight and chemical identity, the Curies undertook the monumental task of processing several tons of the ore residue. The chemical challenge was intensified because radium behaved almost identically to barium, requiring a painstaking separation process. Marie Curie spent over three years performing countless cycles of fractional crystallization, eventually yielding just over one-tenth of a gram of nearly pure radium chloride.
The Concept of Radioactivity
The discoveries of polonium and radium provided the evidence needed to establish the modern scientific concept of radioactivity. Earlier work by Henri Becquerel had shown that uranium emitted rays, but the Curies’ research demonstrated a much deeper principle. Marie Curie found that the amount of radiation emitted was directly proportional only to the quantity of the radioactive element present.
This observation was profoundly significant because it showed that the emission of these rays was a property of the atom itself, not the result of a chemical reaction or molecular interaction. The Curies’ work, particularly with the highly active radium, proved that radioactivity was a fundamental atomic phenomenon. This realization shattered the long-held belief that the atom was indivisible and immutable, ushering in a new era of atomic physics. The Curies coined the term “radioactivity” to describe this spontaneous, energetic emission from the atomic nucleus.