Which scientist discovered the radioactive element, radium?

Concept: Radioactivity is the spontaneous emission of radiation by unstable atomic nuclei. A previously unknown radioactive element is established when a mineral shows far more activity than its known constituents can account for, and that excess activity is concentrated by repeated chemical separation until the concentrate shows a fresh spectral signature of its own.

Application: the ore study that produced radium ran in this exact order.

1. Pitchblende, a uranium ore, was measured in Paris in 1898 with a piezoelectric quartz electrometer and found to be several times more active than its uranium content could explain.

2. The excess activity was followed through successive chemical separations of the ore; the fraction that carried barium retained most of it, showing the activity belonged to something other than uranium.

3. Repeated fractional crystallisation of that barium-bearing fraction concentrated the activity further, and the concentrate showed a new spectral line — the signature of a new element, named radium and announced in December 1898.

4. The work was carried out by Marie Curie together with Pierre Curie; Marie Curie went on to isolate radium as a metal in 1910 and received the 1911 Nobel Prize in Chemistry for radium and polonium.

Cross-check: the other names belong to different fields and, in two cases, to centuries before radioactivity was known.

• Albert Einstein — the photoelectric effect and relativity, twentieth-century theoretical physics, not the chemical isolation of a new element.

• Benjamin Franklin — eighteenth-century atmospheric electricity and the lightning rod, roughly a century before radioactivity was observed.

• Isaac Newton — seventeenth-century mechanics, gravitation and optics, also long before radioactivity was observed.

Result: the scientist who discovered radium is Marie Curie, working with Pierre Curie.