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Radius: structure, properties, uses, obtaining

It is an element surrounded by incredible and regrettable stories, all as a result of the ignorance of its negative effects on health. The bluish glow of its salts and aqueous solutions captivated the scientists Pierre and Marie Curie, who discovered it in 1898 by analyzing inexplicable and strong radiation from samples of the mineral pitchblende, which had had their uranium content removed and polonium.

Illustration of Marie Curie, discoverer of radium

With the uranium and polonium removed, the Curies and concluded that there was another element present in the pitchblende, responsible for the strong radiation, which they were ingeniously able to quantify. This new element was chemically very similar to barium; however, unlike its spectral green lines, this one was crimson.

Thus, after processing, purifying and analyzing tons of pitchblende, they obtained 0.1 mg of RaCl 2 . By then, in 1902, they had already determined the atomic mass of radius, whose name was derived from the Latin word ‘radius’, which means ‘lightning bolt’. Radius is literally a radiant element.

Structure

Radium is a metal whose atoms are very voluminous. A consequence of this is that its crystals are not very dense (although slightly more than barium), and they adopt a cubic structure centered on the body (bcc). Outside of this structure, no other allotropes are reported at other temperatures or pressures.

Electronic configuration

The electron configuration for the radio is as follows:

[Rn] 7s 2

It is extremely easily oxidized to the Ra 2+ cation , which is isoelectronic to the noble gas radon. Therefore, all radium compounds contain the Ra 2+ cation , establishing predominantly ionic interactions; however, it is calculated that it is possible that, due to relativistic effects, it can form bonds with a remarkable covalent character (Ra — X).

Radius properties

Compound sample with radius

Physical appearance

Radium is a silvery-white metal, which immediately turns yellow and darkens when it reacts with nitrogen in the air to form its nitride, Ra 3 N 2 . Because it is so difficult to manipulate, few physical characteristics of the radio have been directly determined.

88

Molar mass

226.03 g / mol

Melting point

Around 700 ºC. This magnitude could not be determined exactly.

Boiling point

The radius boils between 1100 and 1700 ºC. This large margin of inaccuracy reflects, again, the complications of handling such an unstable metal.

Density

5.5 g / cm 3

Heat of fusion

8.5 kJ / mol

Heat of vaporization

113 kJ / mol

Oxidation state

Radium has a unique oxidation state of +2.

Electronegativity

0.9. This value corresponds closely to the strong electropositive character of the radius.

Ionization energies

First: 509.3 kJ / mol

Second: 979 kJ / mol

Radioactivity

Radium is derived from radioactive decays of heavier and more unstable elements, such as the isotope uranium-238. Source: User: Tosaka, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons

Radium is about a million times more radioactive than uranium. This is mainly due to the fact that other isotopes are produced from it, such as 222 Rn (upper image), which in turn decays, emitting α particles to become the 218 Po isotope . A radio sample therefore emits large amounts of α and β  particles , as well as γ rays.

For example, a gram of radium undergoes 3.7 · 10 10 decays per second, a measure that served to define the unit Ci, called curie. This energy is sufficient to heat a sample of 25 grams of water 1 ° C per hour.

If the chain of decays is observed, it will be seen that the initial isotope, 238 U, has a 1/2 of 4.5 Giga years (billions of years); On the other hand, 226 Ra has a 1/2 of 1600 years, which also produces isotopes with 1/2 of just days, minutes and seconds.

The radioactivity of radium and its compounds is ionizing, so that fluorescent substances, even the atoms that surround them, light up at night, becoming capable of triggering explosive reactions. This property is known as radioluminescence.

Compounds and reactivity

When salts or radium compounds are heated in a lighter they give off a carmine-colored flame. Such solids are generally white or colorless, but they turn yellow and eventually darken as a result of the radioactivity of the radio atoms. If they have barium impurities, they may show pinkish tones.

The chemistry of radium is very similar to that of barium, as if they were two inseparable brothers, Ra-Ba. Both form the same salts, insoluble in water, with the difference that those of radium are a little more insoluble. For example, radium sulfate, RaSO 4 , is more insoluble than barium sulfate, BaSO 4 ; in fact, it is the most insoluble sulfate ever known: 2.1 mg of it dissolves in just 1 liter of water.

On the other hand, radio hydroxide, Ra (OH) 2 , is the most soluble and basic of all the hydroxides of its congeners.

In solution, the Ra 2+ cations , instead of behaving like Lewis acids, are basic, since their large size makes it impossible for the molecules to arrange themselves around them to complex or coordinate them.

The chemical analogy between radium and barium makes this metal difficult to separate, because its salts coprecipitate with those of barium.

Uses / applications of radio

Luminous watches

Luminous and phosphorescent clocks were highly desired in the 19th century, for their visual appeal and for containing the acclaimed radio in their paintings. Source: Arma95, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

The most popular and controversial use of radium was as an additive to paint on watch faces. Its atoms were incorporated into pasty formulations of zinc sulfide, ZnS, which absorbed radioactive emissions to give off a green, phosphorescent light. The paint was applied on the hands of the clocks (top image).

The problem with this use lay in the continuous radioactive exposure that affected the users of these watches; or worse still, the workers who painted them in the factories.

In 1924, several female employees who worked in a painting industry, later known as the Radium Girls, began to suffer from bone cancer, deformation of their jaws, and loss of teeth, due to having to lick the tip. of their brushes to be able to usefully apply the radioactive paint on the clocks.

This, plus the ignorance of the radio, and the unethical of not imposing protection measures that would guarantee the safety of the workers, ended in a legal scandal that revolutionized the laws of occupational safety.

Radio paints were discontinued in 1960; although today there are several collectible copies that have it on their surfaces.

Radithor

Radithor flask exhibited at the National Museum of Nuclear Science and History, New Mexico. Source: Sam LaRussa from United States of America, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons

Radithor was one of many pharmaceutical and cosmetic products that touted the efficiency of radium in fighting a thousand and one evils. It contained a micrometric amount of radium, which was supposed to cure all digestive and endocrine diseases. However, in 1932, after the death of Eden Byers, one of its most ardent consumers, the Radithor went off the market.

Radiotherapy

Not all uses of radio were sensational, pseudoscientific, and unscrupulous (or criminal). There were also really beneficial ones for health, at least up to a certain time. The radium salts, RaCl 2 and RaBr 2 , are used in the treatment of cancer, specifically bone cancer.

The 223 Ra isotope , being confused by calcium ions in metabolism, ends up destroying cancer cells located in the bone.

However, many of the therapeutic uses of radium have been replaced by safer, cheaper and more efficient isotopes, such as 60 Co and 137 Cs.

Obtaining

Radium is one of the elements whose production is scarce: just about 100 grams a year worldwide. Tons of uranium ores, such as the mineral pitchblende (or uraninite) are processed following the Curie method, which consists of treating the samples with sodium hydroxide, hydrochloric acid and sodium carbonate, in such a way that a mixture of radio and barium sulfates, RaSO 4 -BaSO 4 .

The RaSO 4 -BaSO 4 mixture was transformed into their respective chlorides, RaCl 2 -BaCl 2 , the cations Ra 2+ being separated by fractional crystallizations and finally purifying using ion chromatography techniques. RaCl 2 is reduced by electrolysis, or by reducing it with metallic aluminum at 1200 ºC.

Isotopes

All isotopes of radium are radioactive. Four of them exist in nature as products of radioactive decay of 232 Th, 235 U, and 238 U atoms .

These four isotopes are: 223 Ra ( 1/2 = 11.4 days), 224 Ra ( 1/2 = 3.64 days), 226 Ra ( 1/2 = 1600 years) and 228 Ra (5.75 years). Obviously, almost all radium atoms consist of mixtures of the 226 Ra and 228 Ra isotopes , since the others disintegrate rapidly.

The 226 Ra is the most stable of all isotopes of radium, being all others too unstable, with 1/2 less than two hours.

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