Astatine and Francium

Of course. Friend was not entirely original in choosing the direction of his search. As early as the end of the 19th century a chemist would not hesitate to answer the question 

But all the attempts to hnd eka-iodine and eka-cesium failed and efiorts of Friend were no exception. 

Now let us turn back to the last decades of the 19th century. When Mendeleev developed the periodic system of elements it contained many empty slots corresponding to unknown elements between bismuth and uranium. These empty slots were rapidly filled after the discovery of radioactivity. Polonium, radium, radon, actinium, and finally protactinium took their places between uranium and thorium. Only eka-iodine and eka-cesium were late. This fact, however, did not particularly trouble scientists. These unknown elements had to be radioactive since there was not even a hint of doubt that radioactivity was the common feature of elements heavier than bismuth. Therefore, sooner or later radiometric methods would demonstrate the existence of elements 85 and 87. 

The natural isotopes of uranium and thorium in long series of successive radioactive transformations give rise to secondary chemical elements. In the first decade of the 20th century scientists had in their disposal about forty radioactive isotopes of the elements at the end of the periodic system, that is, from bismuth to uranium. These radioelements comprised three radioactive families headed by thorium-232, uranium-235 and uranium-238. Each radioactive element sent, its representatives to these families with the only exception of eka-iodine and eka-cesium. None of the three series had links that would correspond to the isotopes of element 85 or 87. This suggested an unexpected idea that eka-iodine and eka-cesium were not radioactive. But why Nobody dared to answer this question. Under this assumption it was meaningless to look for these elements in the ores of uranium and thorium which contained all the radioactive elements without exception. 

The assumption about stability of eka-iodine and eka-cesium was not confirmed. But all eSorts to find isotopes 

---

Lavrukhina, A. K., Pozdnyakov, A. A. Analytical Chemistry of Technetium, Promethium, Astatine, and Francium. Ann Arbor Humphrey Science Publishers 1970... 

Radon, the heaviest of the noble gases, has been much publicized in recent years because of a fear that low-level exposures increase the risk of cancer. Like astatine and francium, its neighbors in the periodic table, radon is a radioactive element with only a minute natural abundance. It is produced by radioactive decay of the radium present in small amounts in many granitic rocks, and it can slowly seep into basements, where it remains unless vented. If breathed into the lungs, it can cause radiation damage. 

The heaviest natural element is uranium, number 92. But gaps in the Periodic Table of 1925 reveal four elements not accounted for. These four missing elements are numbers 43, 61, 85, and 87, now known as technetium, promethium, astatine, and francium. Some of them were reportedly discovered before 1937, but these "discoveries proved to have been erroneous. 

These related elements will carry the traces of radium from the solution, and it is by methods such as this that the chemical identification of technetium, promethium, astatine, and francium were made. 

Astatine and francium are formed from uranium only in most minute quantities, the scarcity explaining why they were not discovered earlier. Technetium and promethium are formed in even smaller quantities, and are unusual in that they are the elements of atomic number less than 84 which sess no stable isotopes at all. 

In the radioactive families the isotopes of astatine and francium are placed not on the principal pathways of radioactive transformations but at the side branches. Here is the branch on which natural francium is born ... 

Thirteen neptunium isotopes are currently known. One of them (neptunium-237) was found in 1952 in nature. This is another example when a previously synthesized element was found in nature and for which two discovery dates can be given (as for technetium, promethium, astatine, and francium). 

Chemistry of the chain elements. Astatine and francium were discovered as the products of nuclear reactions. They are present in only one of the natural chains and even then not in the main branch (seeO Fig. 13.3). The masses given in the last three columns refer to undisturbed (secular] equilibrium along the whole chain. See text for more details... 

Lavmkhina AK, Pozdnyakov AA (1%6) Kondor R (trans) (1970) Analytical chemistry of technetium, promethium, astatine and francium. Arm Arbor-Humphrey, Arm Arbor... 

Taking francium as an example, it was assumed that the minute traces of francium ion Fr could be separated from other ions in solution by co-precipitation with insoluble caesium chlorate (VII) (perchlorate) because francium lies next to caesium in Group lA. This assumption proved to be correct and francium was separated by this method. Similarly, separation of astatine as the astatide ion At was achieved by co-precipitation on silver iodide because silver astatide AgAt was also expected to be insoluble. 

Astatine Astatine, like francium in group 1 A, is a radioactive element that occurs only in minute amounts in nature. No more than about 5 X 10-8 g has ever been prepared at one time, and little is known about its chemistry. 

During World War II, more work was done on the elements, particularly as part of the Manhattan Project. After the war, the existence of these new elements was made public. All of the naturally occurring elements from hydrogen to uranium, including the small number of artificially created but low-number elements francium, astatine, and promethium, had been found and put into the periodic table. Combined with the work of Rutherford and Bohr, this work made the physical structure of the atom clear and proved beyond all doubt. Even if physics would start to find smaller parts within the electron, the neutron, and the proton, from a chemical point of view, the atom was complete. Atoms and molecules had been shown to be real, they way they combined was... 

Many nuclear reactions have been induced by such bombardment techniques. At the time of development of particle accelerators, there were a few gaps among the first 92 elements in the periodic table. Particle accelerators were used between 1937 and 1941 to synthesize three of the four missing elements numbers 43 (technetium), 85 (astatine), and 87 (francium). 

Pm), astatine (At), and francium (Fr) are included here although they actually were given their names later, and some were discovered at a later date. 

One of the most interesting results of bombardment is the creation of completely new elements. Four of these elements, produced between 1937 and 1941, filled gaps in the periodic table for which no natnrally occurring element had been found. These four are technetium (Tc, number 43), promethium (Pm, number 61), astatine (At, number 85), and francium (Fr, number 87). The equations for the reactions for their production are ... 

Most of the elements found in nature have several isotopes. Elements with atomic number 83 (bismuth) and lower have at least one stable isotope, although some are also radioactive and unstable. From element number 84 (polonium) and upwards, all the elements lack stable isotopes. The nine elements 84-92 are called naturally occurring radioactive elements. They are polonium, astatine, radon, francium, radium, actinium, thorium, protactinium and uranium. They are all treated in this chapter. There are also an additional two radioactive elements, number 43 technetium and 61 promethium. However, they have been described in Chapter 28 Technetium and Chapter 17 Rare earths, respectively. 

Although there are a number of isotopes of francium, most decay very rapidly to other elements. Most isotopes with masses of 223 AMU and lower emit tt-particles (consisting of two protons and two neutrons) to become astatine. Some low mass francium isotopes can also undergo electron capture (the conversion of a proton to a neutron through the absorption of an electron) to become radon. Francium isotopes with masses of 220 AMU and higher can undergo /3-decay (the conversion of a neutron to a proton through the emission of an electron) to become radium. Francium-223 is the most stable isotope and has a half-life of 21.8 minutes. 

From their positions in the periodic table, predict, as accurately as you can, some physical and chemical properties of francium and astatine. 

---