Radioactivity — Marie Curie Discovers Radium and Polonium

Discovery

Technical Explanation

1. The initial anomaly — In 1897, Marie Curie studies "Becquerel rays" (discovered in March 1896). By measuring the ionization of air by various uranium ores with Pierre Curie's piezoelectric electrometer, she observes that pitchblende emits radioactivity 4× greater than pure uranium. The hypothesis is inescapable: the ore contains an unknown, more active element.

2. Separation by fractional chemistry — Marie Curie processes the ore through successive acid attacks (HCl, H₂SO₄, NaOH), then separates the fractions by selective precipitation. At each step, she measures the radioactivity of each fraction with the electrometer. She identifies two abnormally active fractions: one co-precipitating with bismuth (polonium) and one with barium (radium).

3. Fractional crystallization of radium — To isolate radium from barium (nearly identical chemistry), Marie performs thousands of fractional crystallizations of the mixed chloride BaCl₂/RaCl₂. Radium chloride, being slightly less soluble, concentrates in the crystals. After 4 years of work (1898–1902), she obtains 0.1 g of pure radium chloride and determines its atomic mass: 225 (current value: 226.03).

4. Proof by spectroscopy — Eugène Demarçay confirms radium's existence through emission spectroscopy: the line at 381.47 nm, absent from barium's spectrum, is unique to radium. This spectral line constitutes irrefutable proof of a new element.

Why It Worked

Marie Curie's method relies on a quantitative measuring instrument: the piezoelectric electrometer invented by Pierre and Jacques Curie (1880). This device measures ionization current with a precision of 10⁻¹² amperes. At each chemical step, Marie measures the fraction's radioactivity to know whether she is getting "warmer" or "colder" — navigation by measurement, not by intuition.

Physical perseverance was equally decisive. The shed on rue Lhomond had no heating, no ventilation, and no fume hood. Marie stirred pitchblende residues in cast-iron cauldrons in open air — under conditions that likely cost her her life (she died of aplastic anemia in 1934).

Causal Chain

Röntgen discovers X-rays (November 1895) → Becquerel discovers uranium radioactivity (March 1896) → Marie Curie identifies the pitchblende anomaly (1897) → Discovery of polonium and radium (1898) → Nobel Prize in Physics (1903, shared Curie/Becquerel) → Rutherford discovers the atomic nucleus (1911) → Nobel Prize in Chemistry for Marie Curie alone (1911) → Chadwick discovers the neutron (1932) → Hahn/Strassmann achieve fission (1938) → Nuclear age

Anecdote

Marie Curie's laboratory notebooks, held at the Bibliothèque nationale de France, remain radioactive over 120 years later. Consulting them requires protective clothing and the signing of a liability waiver. The contamination is primarily due to radium-226, which has a half-life of 1,600 years, and to polonium. Marie's fingerprints are still visible on certain pages — imprinted in radium and polonium.

Sources

References verified during the August 2026 fact-checking audit: these are the pages
against which this bulletin's claims were checked.

1. Marie Curie and the Science of Radioactivity — American Institute of Physics
2. This Month in Physics History: Curie discovery of radium — APS News
3. Marie and Pierre Curie and the discovery of polonium and radium — NobelPrize.org