Radium · element 88
The glowing element that made the Curies famous, painted onto watch dials, and poisoned the women who painted them
Key facts
For a few decades in the early twentieth century, radium was the most glamorous substance on Earth. It glowed in the dark, it seemed to cure cancer, and it cost more per gram than anything ever sold. It was put into watch dials, toothpaste, face cream and drinking water. Then the people who worked with it began to sicken and die, and radium became the first great lesson in the dangers of radioactivity.
Radium is a silvery white metal in group 2, a heavier cousin of calcium and barium. Its most common isotope, radium-226, has a half-life of 1,600 years, long enough to be a persistent hazard and short enough to be intensely radioactive.
Where it is found
Radium is a decay product of uranium, so it occurs in all uranium ores, but only in tiny proportions: roughly one gram for every three tonnes of uranium. The Curies extracted their first radium from pitchblende mined at Joachimsthal in what is now the Czech Republic; later supplies came from Colorado, the Belgian Congo and Canada.
Commercial production ended in the mid twentieth century once artificial isotopes replaced radium in medicine, and today small amounts are recovered from spent nuclear fuel and uranium mill waste when needed. Radium also turns up where it is not wanted, in groundwater in some regions and as scale inside oil and gas pipes.
How it was discovered
Five months after finding polonium, Marie and Pierre Curie, with the chemist Gustave Bémont, announced radium in December 1898. A fraction of pitchblende that followed barium through their separations was far more radioactive than it should have been, and its spectrum showed lines belonging to no known element. They named it from the Latin radius, a ray.
Proving it existed took another four years. Marie Curie processed tonnes of ore residue in an unheated shed to extract a tenth of a gram of radium chloride and measure its atomic mass. The work earned the Curies a share of the 1903 Nobel Prize in Physics, and Marie a second prize, in chemistry, in 1911. In 1910 she and André Debierne isolated the pure metal by electrolysis.
What we use it for, and used to
Radium's first use was medical. Sealed needles and tubes of radium were placed in or beside tumours, and radium therapy remained a mainstay of cancer treatment until reactor made isotopes such as cobalt-60 replaced it in the 1950s and 1960s. Radium mixed with zinc sulfide made luminous paint for watch dials, aircraft instruments and gun sights.
Then came the quackery. Radium water, chocolate and toothpaste were all sold as tonics, and the American industrialist Eben Byers, who drank a radium tonic daily, died in 1932 as his jaw disintegrated. Today radium has one modern medical role: radium-223, which behaves like calcium and homes in on bone, was approved in 2013 to treat prostate cancer that has spread to the skeleton.
Reading the numbers
Radium melts at about 960 °C and boils near 1740 °C, similar to barium above it. Its density of 5.5 grams per cubic centimetre is surprisingly low for such a heavy atom, less than iron, because group 2 atoms are large and loosely packed. The atomic mass of 226 is the mass number of the longest lived isotope, since radium has no stable ones.
The electron configuration [Rn] 7s2 has two electrons outside a radon core, the defining feature of group 2, the alkaline earth metals. Radium gives them up readily, with ionisation energies of 509 and 979 kJ/mol, to form Ra2+ ions that behave almost exactly like calcium and barium ions, and its electronegativity of 0.9 is very low. This resemblance to calcium is the reason radium is so dangerous: the body mistakes it for calcium and builds it into bone. Radium sits in period 7, directly below barium.
Safety and the environment
Swallowed or inhaled radium lodges in bone and stays for years, bombarding marrow and bone with alpha particles. The results are anaemia, fractures, bone cancers and the destruction of the jaw seen in the dial painters, who had been told to point their brushes with their lips. Their lawsuits in the 1920s and 1930s helped establish workers' rights to compensation for occupational disease.
Radium also decays into radon gas, so any radium source slowly leaks radon. Old luminous watches, compasses and aircraft dials are still around in drawers and collections; they are safe to keep intact but should never be opened or scraped. Sites where radium was processed, or where wartime aircraft dials were dumped and burned, such as Dalgety Bay in Scotland, have needed decades of clean up.
Did you know?
- The paint on old radium watch dials has stopped glowing not because the radium is gone, only about five per cent has decayed in a century, but because the zinc sulfide phosphor has been worn out by the radiation.
- Marie Curie's laboratory notebooks are still radioactive and are stored in lead lined boxes; readers must sign a waiver to handle them.
- In the 1920s radium sold for more than 100,000 US dollars a gram, making it the most expensive substance in the world.
- The radium dial painters' court cases in the United States were among the first to establish that workers could sue employers for occupational disease.
- Radium-223 was approved in 2013 as a treatment for prostate cancer that has spread to bone.
- Pure radium glows faintly blue in the dark and is warm to the touch, both effects of its own radioactivity.
More in the alkaline earth metals
These share a column with radium, so their outer electrons are arranged in a similar way and they tend to behave alike.
See all 6 alkaline earth metals on one page, or browse all 118 elements.
Compare Radium with another element
Each comparison has a side-by-side data table and a plain-English summary of the differences.
Frequently asked questions
Does radium still glow?
The radium in old dials is still almost entirely present and still radioactive, but the glow came from a zinc sulfide phosphor excited by the radiation, and that phosphor has degraded. So old radium dials are usually dark yet still radioactive, which is the opposite of what most people assume.
Is it safe to own a radium watch?
An intact vintage radium watch gives off a small amount of radiation and some radon, and is generally considered safe to keep and even wear occasionally. It should never be opened, scraped or repaired by an amateur, because radium paint dust is dangerous if inhaled or swallowed.
Why is radium so dangerous?
Radium behaves chemically like calcium, so the body deposits it in bone, where it stays for years. Its alpha particles then damage bone and marrow, causing anaemia, fractures and cancer. It also produces radon gas, which adds to the exposure.
How did Marie Curie discover radium?
She and Pierre Curie noticed that pitchblende ore was more radioactive than its uranium content could explain. By dissolving the ore and separating it chemically, they found the excess radioactivity stayed with barium, and its spectrum revealed a new element, which they announced in December 1898.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Radium reference article. Spotted an error? Tell us.