Radium
Radium is a chemical element with symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is almost colorless, but it readily combines with nitrogen (rather than oxygen) on exposure to air, forming a black surface layer of radium nitride (Ra3N2).
Identity
- Atomic mass
- 226 u
- Category
- Alkaline earth metals
- Group
- 2
- Period
- 7
- Block
- s
- Oxidation States
- +2
- Phase (25 °C)
- Solid
- Appearance
- silvery white metallic
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2
- Short configuration
- [Rn] 7s2
- Electrons per shell
- 2, 8, 18, 32, 18, 8, 2
- Electronegativity (Pauling)
- 0.900
- Electron affinity
- 9.65 kJ/mol
- 1st ionization energy
- 509 kJ/mol
- Ionization energies
- 509 · 979 kJ/mol
Physical properties
- Melting point
- 959.9 °C (1233 K)
- Boiling point
- 1736.8 °C (2010 K)
- Density
- 5.5 g/cm³
- Molar heat capacity
- —
Chemical behavior of radium
Radium possesses two valence electrons in the 7s orbital ([Rn] 7s²). It is the heaviest member of group 2 (alkaline earth metals) and manifests exclusively the divalent Ra²⁺ oxidation state, originating strongly ionic bonds. Chemically it behaves almost indistinguishably from barium, although more electropositive, basic and reactive than this. When freshly prepared it is a bright silvery white metal that quickly tarnishes in air through nitriding and oxidation, and emits a faint bluish luminescence in the dark due to radioluminescence produced by exciting the surrounding air.
Reactivity
Ra + 2 H₂O → Ra(OH)₂ + H₂
It reacts energetically with liquid water releasing hydrogen gas and forming an alkaline solution of radium hydroxide, the strongest base of group 2.
Ra²⁺ + SO₄²⁻ → RaSO₄
It precipitates very easily in the presence of sulfates forming white radium sulfate, the most insoluble salt of all alkaline earth metals.
Ra + O₂ → RaO
It oxidizes rapidly upon contact with air forming radium oxide; at higher temperatures it burns forming radium oxide and peroxide.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Radium chloride | RaCl₂ | First radium salt isolated pure by Marie Curie; historically used as a medical radiation source and activity standards |
| Radium sulfate | RaSO₄ | Extremely insoluble salt used to seal historical radiotherapy sealed sources and separate radium from minerals |
| Radium-223 dichloride | ²²³RaCl₂ | Modern alpha-emitting radiopharmaceutical (Xofigo) approved to treat painful bone metastases of castration-resistant prostate cancer |
| Radium bromide | RaBr₂ | Luminescent crystalline salt that glowed in the dark and served in the first nuclear disintegration experiments |
Electron shell distribution
View in the atom builderIsotopes
All radium isotopes are radioactive, adding up to more than thirty characterized nuclides. The most relevant and long-lived is radium-226 (²²⁶Ra, with 138 neutrons and a half-life of 1,600 years), a uranium-238 descendant that decays emitting alpha particles towards radon-222 gas. Other natural isotopes of medical and geological importance are radium-228 (²²⁸Ra, half-life of 5.75 years, thorium series) and radium-223 (²²³Ra, half-life of 11.4 days, used in alpha oncology therapy).
Occurrence and production
It is a scarce but omnipresent element as a transient decay product in all uranium and thorium ores. In one tonne of pitchblende (uraninite) ore an average of barely 0.14 grams of radium is found in secular equilibrium. There are no primary deposits of pure radium. Historically it was extracted from uranium ores of Jáchymov (Czech Republic), Shinkolobwe (Democratic Republic of the Congo) and Great Bear Lake (Canada) through laborious fractional coprecipitation processes with barium.
History and origin of the name
It was discovered in Paris in December 1898 by scientists Marie Skłodowska-Curie and Pierre Curie, together with Gustave Bémont, upon finding that residual pitchblende was four times more radioactive than the pure uranium it contained. After years of exhausting physical work processing tonnes of ore in a damp shed, Marie Curie and André-Louis Debierne isolated pure metallic radium in 1910 through electrolysis of radium chloride over a mercury cathode. Its name derives from the Latin word 'radius' (ray), owing to the brightness of its radioactive emissions.
Applications and relevance of radium
Uses of the element
In the first half of the 20th century it was massively used mixed with zinc sulfide to produce radioluminescent paints that glowed in the dark on military watch spheres, compasses and aviation instruments, and as a neutron source combined with beryllium. Today, its use in luminous paint is strictly prohibited worldwide.
Uses of its compounds
In contemporary medicine, radium-223 dichloride (Xofigo) is a revolutionary targeted therapy: by mimicking calcium, it selectively attaches to bone metastases and emits short-range alpha particles that destroy tumor cells while preserving healthy adjacent bone marrow.
Health, safety and environment
Biological role
It lacks biological function. Due to its extraordinary chemical affinity with calcium, the ingested Ra²⁺ ion is transported by the blood and permanently deposited in the bone mineral matrix ('bone seeker'), where continued alpha radiation destroys bone marrow causing aplastic anemia, mandibular osteonecrosis and malignant osteosarcomas.
Safety
The tragic story of the 'radium girls' in the 1920s (workers who sharpened radium paint brushes with their lips) led to strict radiological protection standards.
Environment
There are no routine spills: traces are fixed in solid matrices treated as radioactive waste and vitrified.
Frequently asked questions about Radium
Why did the dials of ancient clocks glow in the darkness with radium paint?
After the discovery of radium, its intense radioactivity fascinated the public and the industry. Watchmakers began mixing small amounts of radium salts with zinc sulfide to create self-luminous dials that glowed in the dark without any external light source. The phenomenon was so striking that luminous alarm clocks, compasses and aircraft instruments spread rapidly. The practice was later completely prohibited worldwide after documentation of the severe damage it caused to the workers who manufactured them.
Why do radium salts glow in the dark by themselves?
Unlike common phosphorescent materials that need to be 'charged' beforehand under a light bulb, radium salts shine continuously day and night. The alpha and beta radiation emitted by their atoms collides with nitrogen molecules in the surrounding air, exciting them electrically. When de-exciting, nitrogen emits a faint bluish luminescence visible in total darkness.
What did the pseudoscientific fashion of radium waters and cosmetics in the 1920s consist of?
After its discovery, the popular fascination with atomic energy unleashed a wave of uncontrolled miraculous radioactive products. Ceramic jars lined with radium were commercialized to 'reactivate' daily drinking water, toothpastes, rejuvenating face creams and ointments promising vitality and cure for any disease. The fashion collapsed when public figures such as millionaire Eben Byers died disfigured by bone necrosis after consuming thousands of bottles of radioactive water.
Why does radium fix in bones and cause so much damage to the body?
Radium belongs to the same chemical family as calcium (the alkaline earth metals) and shares similar properties. If ingested or inhaled, the organism confuses it with calcium and transports it directly to the bone structure, where it integrates permanently. Once there, it emits continuous radiation that destroys bone marrow and induces serious tumors without the possibility of being easily eliminated.
What did the tragic legal battle of the 'radium girls' in the United States consist of?
In the 1920s, hundreds of young women worked painting military watch spheres in factories in New Jersey and Illinois. Their supervisors ordered them to sharpen the brushes with their lips, assuring them that the paint was harmless. Over the years they ingested lethal amounts of radium that lodged in their bones, causing jaw necrosis and bone cancers. Their historic lawsuit established modern labor rights and industrial safety standards.
Why does radium continuously generate the dangerous radioactive gas radon in basements and mines?
The radium-226 nucleus disintegrates emitting an alpha particle to become radon-222 with a half-life of sixteen hundred years. Unlike radium, which is a solid metal trapped in the mineral rock, the resulting radon is an inert monoatomic noble gas extremely volatile. The radon gas easily filters through cracks in granite and concrete foundations, accumulating in poorly ventilated basements where it is inhaled as a lung carcinogen.
How did Marie Curie isolate a decigram of radium chloride from tonnes of pitchblende?
Marie Curie obtained tonnes of residual pitchblende slag from the Joachimsthal mines donated by the Austrian government. In a shed without thermal insulation, she boiled the leachates with acids and caustic soda in iron vats for months. Since radium was chemically a twin of barium, she applied an exhaustive fractional crystallization technique: evaporating thousands of times hot chloride solutions, she managed to crystallize radium chloride slightly before barium, obtaining pure radium.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.