Radium – facts and properties
- Symbol
- Ra
- Atomic number
- 88
- Atomic mass
- 226 u
- Period
- 7
- Group
- 2
- Category
- alkaline earth metal
- Electron configuration
- [Rn] 7s²
- Electronegativity
- 0.9
- Atomic radius
- 221 pm
- Valence electrons
- 2
- State at standard conditions
- solid
- Melting point
- 700 °C
- Boiling point
- 1140 °C
- Density
- 5 g/cm³
- Discovered
- 1898
Radium is a silvery-white, radioactive alkaline earth metal with atomic number 88. Marie and Pierre Curie discovered it in 1898 in pitchblende and named it after the Latin word radius, meaning ray. Radium glows in the dark, was long regarded as a miracle cure and went into luminous paint, until people realised how dangerous it is inside the body. Today its short-lived isotope radium-223 is used to treat bone metastases in prostate cancer.
Position in the periodic table
Occurrence and extraction
Radium occurs only in minute traces, because it is a decay product of uranium. The most important isotope, radium-226, forms in the uranium-238 decay series and has a half-life of about 1600 years. That is far shorter than the age of the Earth; it survives because it is formed continuously from uranium and is found only where uranium is: in pitchblende, in granite and in some spring and ground waters.
Radium itself decays by alpha emission to the radioactive noble gas radon, which escapes from rock and soil and can build up in buildings.
Pierre and Marie Curie studied pitchblende from Joachimsthal in Bohemia that radiated more strongly than its uranium content explained. From several tonnes of residues they laboriously separated the radium salts from chemically similar barium by repeated crystallisation. Marie Curie and André Debierne prepared the pure metal in 1910. Today radium is hardly extracted any more; it remains in the residues of uranium processing. The radium-223 used in medicine comes instead from the uranium-235 series and is separated from decaying actinium-227.
Properties and reactions
Radium is in period 7 and group 2, below barium. Francium is on its left, and on its right the actinoids begin with actinium. The atom has two valence electrons, its electron configuration is [Rn] 7s², its electronegativity is 0.9 and its atomic radius is 221 pm. Its atomic mass is given as 226 u, the mass number of the longest-lived isotope, radium-226. The metal melts at 700 °C, and its density is 5 g/cm³. These values are known only roughly.
Chemically, radium resembles its lighter relatives in the group, above all barium. It is a base metal that tarnishes quickly in air and releases hydrogen from water. In compounds it forms a doubly charged positive ion. Radium sulfate is among the least soluble sulfates of all, so radium can be precipitated together with barium sulfate.
Radium-226 emits alpha radiation, and its decay products add penetrating gamma rays. Radium salts glow faintly blue because the radiation excites the surrounding air, and they make phosphors shine.
In the body radium behaves like calcium and is built into the bones. There it irradiates bone tissue and marrow for years. The consequences are bone damage and bone cancer.
From luminous paint to cancer therapy
After its discovery radium became a craze. Mixed with zinc sulfide it gave a paint that glowed greenish without any light source, because the radiation excites the zinc sulfide. From the early twentieth century, watch faces and aircraft instruments were painted with it. It was also sold as a remedy, in drinking water, toothpaste and creams. Its benefit was never shown.
The "Radium Girls" were young women who painted dials in American factories from 1917 onwards. To draw fine lines they shaped the brush tip with their lips, which took radium into their bodies and bones. Many developed severe jaw damage and cancer, and some died. In court cases at the end of the 1920s the women won compensation. The case became a turning point for workplace safety law.
The strength of a radiation source was long given in curies. Originally one curie was the activity of one gram of radium-226. Today the unit is the becquerel, one decay per second. Luminous radium paint was later replaced by less hazardous substances.
Medicine today uses the isotope radium-223. It is an alpha emitter with a half-life of about eleven days. Like calcium it accumulates in bone, especially where metastases are remodelling the tissue, and it irradiates them over a range of only a few cell layers. The drug has been approved since 2013 for bone metastases in advanced, castration-resistant prostate cancer.
Quick question
What is the atomic number of radium (Ra)?
FAQ
Why does radium glow in the dark?
The metal itself glows only faintly blue, because its radiation excites the air. The greenish paint on old dials was a mixture of a radium salt and zinc sulfide. The radiation excited the zinc sulfide, and that gave off the light.
Who were the Radium Girls?
They were young women who painted watch dials with radium paint in the early twentieth century and took in radium as they worked. Many fell seriously ill. Their lawsuits led to better workplace and radiation protection.
Is radium still used today?
Only in medicine and research. Radium-223 helps against bone metastases in prostate cancer. Luminous paint, radium baths and other early uses vanished once the dangers were known.