Radioactive brilliant white metal m.p. 700 ºC; 1292 ºF b.p. 1140 ºC; 2084 ºF density 5? g/cc; 312? pound/cubic foot	memory peg
	1898 Pierre & Marie Curie, France
	radius = ray (Latin)

On 26 December 1898, Pierre and Marie Curie announced the discovery of this element. It had been distinguished from Polonium due to the likeliness of its chemical properties with those of Barium. Its sulfate and carbonate were insoluble and the chloride was soluble in water but insoluble in hydrochloride acid or in alcohol. However, this element was not identical to Barium, and could easily be separated. They named it Radium, after the Latin radius = ray, because the radiation is 3 millions times as much as that of Uranium (note).

Some years later, in 1902, Marie Curie performed a series of fractional crystallizations starting from a considerable amount of uraninite residues, and was able to isolate about 0,1 grams of chloride of almost pure Radium, with an activity about 3 million greater than that of uranium.

The announcement of the discovery of Polonium and of Radium triggered a series of research works, leading to the discovery of another radioactive elements associated to Uranium and Thorium.

Isotopes with the historical name Radium-...

Name	Hist. symb.	Mod. symb.
Radium	Ra	226Ra
Radium Emanation	Ra Em	222Rn
Radium-A	Ra A	218Po
Radium-B	Ra B	214Pb
Radium-C	Ra C	214Bi
Radium-C'	Ra C'	214Po
Radium-C''	Ra C''	210Tl
Radium-D	Ra D	210Pb
Radium-E	Ra E	210Bi
Radium-E''	Ra E''	206Tl
Radium-F	Ra F	210Po
Radium-G	Ra G	206Pb

Historical names of Radium Isotopes

Name & Symbol (hist. and modern)			First described		Notes
Mesothorium 1	MsTh1	228Ra	1906/07	B.B. Boltwood	Boltwood's Mesothorium split by O. Hahn in Mesothorium 1 and 2 (=228Ac)
Thorium-X	Th X	224Ra	1902	E. Rutherford & F. Soddy
Actinium-X	Ac X	223Ra	1905	T. Godlewski

Further reading:

• Mary Elvira Weeks, Discovery of the Elements, comp. rev. by Heny M. Leicester (Easton, Pa.: Journal of Chemical Education, 1968), pp. 781-783.
• Radium und Isotope. Gmelins Handbuch der anorganische Chemie, 8. Aufl.; System-Nummer 31 (1928).
• Winfried Kölzer, Radioaktivität, Strahlenexposition, Strahlenwirkung. Bonn: Informationskreis Kernenergie, 2000 (PDF file on-line).

In some thin samples of certain minerals, notably mica, there can be observed tiny aureoles of discoloration which, on microscopic examination, prove to be concentric dark and light circles with diameters between about 10 and 40 mm and centered on a tiny inclusion. These so-called "pleochronic halos" were first reported between 1880 and 1890. Their origin was a mystery until the discovery of radioactivity and its powers of coloration. Scientists in the early 20th century studied these "pleochronic halos" because they are an integral record of radioactive decay in minerals that constitute the most ancient rocks. Most importantly, this thermal-resistant record is detailed enough to allow estimation of the decay energies involved and to identify the nuclides decaying. This latter possibility is particularly exciting because classes of halos exist which correspond to no known radionuclide. Barring the possibility of a nonradioactive origin, these are evidence for hitherto undiscovered or presently extinct radionuclides. John Joly, a geology professor at Dublin, lost nearly all his halo evidence for an element he called Hibernium (after Hibernia, Latin for Ireland) in the Easter uprising of 1916.

Further reading:

• Marco Fontani, Mariagrazia Costa, and Arnaldo Cinquantini, Dagli aloni pleocroici alla nascita della Terra. RICH-MAC Magazine 85, La Chimica e l'Industria, Ottobre 2003, pp. 65-67.
• Robert V. Gentry, Radioactive Halos. Annual Review of Nuclear Science 23 (1973). (on-line).
• D. Weaire and S. Coonan, The parrot, the pince-nez and the pleochroic halo. Europhysics News Vol. 32 No. 2 (2001) (on-line).