Radioactive Elements Polonium

We have already discussed the history of discovery of two natural radioactive elements, that is, uranium and thorium, in Chapter 4. These elements can fairly easily be found in minerals with chemical analysis since their content is sufficiently high. Other natural radioactive elements (polonium, radon, radium, actinium, and protactinium) are among the least abundant elements on Earth. Moreover, they exist in nature only because they are the products of radioactive transformations of uranium and thorium. 

These elements belonging to the end of the periodic system could not be determined either with chemical analysis or with spectroscopic techniques. They were present in all the minerals where uranium and thorium were found. But not once did scientists suspect that uranium and thorium contained some impurities. Of course, there were always impurities but their content was too low to shift the weighing pans of a balance or to give rise to a new spectral line. 

It was only the discovery of a new physical phenomenon known as radioactivity that presented scientists with a method which contributed to a considerable expansion of our knowledge of the properties and structure of matter and to a significant increase in the number of chemical elements in the periodic system. At the early stage of the studies of radioactivity three types of radiation were found alpha rays (fluxes of the nuclei of helium atoms with the positive charge of two), beta rays (fluxes of electrons with the negative charge of one), and gamma rays (these are in fact rays similar to X-rays). 

Each radioactive element is described by its half-life, that is, the time required for half the initial amount of a radioactive substance to become disintegrated. 

Polonium was the first natural radioactive element discovered with the radiometric technique. Back in 1870 the main properties of polonium were predicted by D. I. Men- 

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Lind (1961) defines radiation chemistry as the science of the chemical effects brought about by the absorption of ionizing radiation in matter. It can be said that in 1895, along with X-rays, Roentgen also discovered the chemical action of ionizing radiation. He drew attention to the similarity of the chemical effects induced by visible light and X-rays on the silver salt of the photographic plate. This was quickly followed by the discovery of radioactivity of uranium by Becquerel in 1896. In 1898, the Curies discovered two more radioactive elements—polonium and radium. 

Marie (NLP 1903, NLC 1911 ) and Pierre (NLP 1903 ) Curie took up further study of Becquerel s discovery. In their studies, they made use of instrumental apparatus, designed by Pierre Curie and his brother, to measure the uranium emanations based on the fact that these emanations turn air into a conductor of electricity. In 1898, they tested an ore named pitchblende from which the element uranium was extracted and found that the electric current produced by the pitchblende in their measuring instrument was much stronger than that produced by pure uranium. They then undertook the herculean task of isolating demonstrable amounts of two new radioactive elements, polonium and radium, from the pitchblende. In their publications, they first introduced the term radio-activity to describe the phenomenon originally discovered by Becquerel. After P. Curie s early death, M. Curie did recognize that radioactive decay (radioactivity) is an atomic property. Further understanding of radioactivity awaited the contributions of E. Rutherford. 

The creation, by neutron bombardment of uranium, of the so-called transuraniums is based on the discovery of artificial radioactivity by M. and Mme. Joliot-Curie. Irene Curie was bom in Paris in September, 1897, the elder daughter of M. and Mme. Pierre Curie of honored memory. Both in Poland and in France she had many relatives who were devoting their lives to science, and from her earliest childhood she lived in a scientific atmosphere, among distinguished chemists and physicists. When Irene was less than a year old, her mother discovered the radioactive element polonium, which was destined to play an important part in the later researches of both mother and daughter. A few months later M. and Mme. Curie discovered another element of even greater importance, which they named radium. 

There are three names coimected with the discovery of radioactivity Henry Bec-querel, who discovered this phenomenon in 1896 [2] Maria Sklodowska-Curie, who named this process radioactivity and her husband Pierre Curie [3]. They stated that uranium salts emit ionizing rays and, furthermore, Maria Sklodowska-Curie discovered that thorium gives off the same rays. She proved that radiation was not the outcome of some interaction of molecules, but must come from an atom itself this discovery was absolutely revolutionary. Maria and Pierre discovered the first two radioactive elements, polonium and radium. There are about 20 radioactive elements and about 50 radionuclides in the natural environment. 

Over the next decade, many scientists worked to find out more about radioactive materials. Curie and her husband, Pierre Curie (1859-1906), isolated two new radioactive elements, polonium and radium. In 1900, German physicist Friedrich Ernst Dorn (1848-1916) found a third radioactive element radon. 

OF ORE TO A FEW GRAMS OF RADIOACTIVE SUBSTANCE. QHEY SUCCEEDED IN SHOWING THAT THE ORE CONTAINED TWO NEW RADIOACTIVE ELEMENTS, POLONIUM AND RADIUM. 0Y A LONG SERIES OF LABORIOUS FRACTIONAL CRYSTALLIZATIONS, THEY FINALLY ISOLATED A SMALL QUANTITY OF PURE RADIUM SALT. 

Radioactivity is emitted from atomic nuclei that are unstable and spontaneously change their structure. In 1896, Henri Becquerel first discovered radioactivity when he placed a piece of zinc uranyl sulfate wrapped in paper on a photographic plate. Two years later, Marie and Pierre Curie discovered two highly radioactive elements, polonium and radium, in pitchblende. a particles, the nuclei of helium atoms, were among the radiations emitted by these substances which were spontaneously transmuting. Indeed, since the earth had billions of years ago lost its original complement of light, inert helium, all helium in our... 

Marie and Pierre Curie discover the radioactive elements polonium and radium. Their work establishes the early framework for the study of nuclear chemistry. 

The number of naturally radioactive elements on Earth is considerably smaller than that of stable ones. In the periodic table they begin with polonium (Z — 84) and end with uranium (Z = 92). Among them only thorium and uranium have very long half-lives therefore, they have survived on Earth since the time of its formation and their amounts are rather noticeable. That is why uranium and thorium have been discovered as new chemical elements long before scientists succeeded in observing radioactivity. The amounts of other naturally radioactive elements (polonium radon, radium, actinium, and protactinium) are much smaller,... 

The third place is held by France where fifteen elements were discovered chromium (1797), beryllium (1798), boron (1808), iodine (1811), bromine (1826), gallium (1875), samarium (1879), gadolinium (1886), dysprosium (1886), radium (1898), polonium (1898), actinium (1899), europium (1901), lutecium (1907), francium (1939). It is not surprising that the radioactive elements polonium, radium, and actinium were discovered by French scientists. These discoveries proceeded from the pioneering studies of radioactivity conducted in France. A brilliant spectral analyst P. Lecoq de Boisbaudran discovered by means of spectral analysis four new elements—gallium and three rare-earth elements (samarium, gadolinium, and dysprosium). Chromium and beryllium were discovered by L. Vauquelin who was such a skillful analytical chemist that it would be unjust if he had not given the world at least one new element. 

Several great scientific discoveries were made in a period of a few years, beginning in 1895. These discoveries made great changes in chemistry as well as in physics. X-rays were discovered in 1895, radioactivity was discovered in 1896, the new radioactive elements polonium and radium were isolated in the same year, and the electron was discovered in 1897. 

In 1930 two scientists, W. Bothe and H. Becker, observed that when a-particles in the radiation from the radioactive elements polonium or radium penetrated beryllium a previously unknown radiation was produced. The charge of the particles in this new radiation was examined in a cloud chamber, but no tracks were formed. Thus, the new particles had no charge. James Chadwick in 1932 solved the problem by proposing a particle having a mass about the same as the proton but without a charge. This particle he called the neutron. The reaction that produced the neutron was ... 

X-rays fascinated the scientific community and the general public alike. Scientists investigated further and soon found related phenomena. Chief among these was a discovery made by Antoine-Henri Becquerel, a professor of physics in Paris. While studying the characteristics of fluorescent materials in March 1896, he discovered that uranium exposed a photographic plate when placed next to it. He had found another form of radiation, one that was emitted spontaneously from a natural substance. To investigate further, Becquerel collaborated with a husband-and-wife scientific team, Pierre Curie and Maria Sklodowska-Curie. Sklodowska-Curie devised an electrometer that could measure this radioactivity, as she called it, and studied a number of materials. In 1898 the trio managed to separate from uranium ore two radioactive elements polonium and radium. 

An alplia p uticle is an energetic helium nucleus. The alplia particle is released from a radioactive element witli a neutron to proton ratio tliat is too low. The helium nucleus consists of two protons and two neutrons. The alplia particle differs from a helimn atom in that it is emitted witliout any electrons. The resulting daughter product from tliis tj pe of transformation lias an atomic number Uiat is two less tluin its parent and an atomic mass number tliat is four less. Below is an e. aiiiple of alpha decay using polonium (Po) polonium has an atomic mass number of 210 (protons and neutrons) and atomic number of 84. 

The isolation and identification of 4 radioactive elements in minute amounts took place at the turn of the century, and in each case the insight provided by the periodic classification into the predicted chemical properties of these elements proved invaluable. Marie Curie identified polonium in 1898 and, later in the same year working with Pierre Curie, isolated radium. Actinium followed in 1899 (A. Debierne) and the heaviest noble gas, radon, in 1900 (F. E. Dorn). Details will be found in later chapters which also recount the discoveries made in the present century of protactinium (O. Hahn and Lise Meitner, 1917), hafnium (D. Coster and G. von Hevesey, 1923), rhenium (W. Noddack, Ida Tacke and O. Berg, 1925), technetium (C. Perrier and E. Segre, 1937), francium (Marguerite Percy, 1939) and promethium (J. A. Marinsky, L. E. Glendenin and C. D. Coryell, 1945). 

Although the Curies noted that one equivalent gram of radium released one hundred calorics of heat per hour, they were uninterested in the practical implications of this, as they were both devoted to pure scientific discovery. During their work with pitchblende in 1898, the Curies discovered two new radioactive elements, which they named polonium (in honor of Marie s homeland) and radium. By 1902 they had isolated a pure radium salt and made the first atomic weight determination. 

In 1898, Marie and Pierre Curie isolated two new radioactive elements, which they named radium and polonium. To obtain a few milligrams of these elements, they started with several tons of pitchblende ore and carried out a long series of tedious separations. Their work was done in a poorly equipped, unheated shed where the temperature reached 6°C (43°F) in winter. Four years later, in 1902, Marie determined the atomic mass of radium to within 0.5%, working with a tiny sample. 

One curious observation, however, was that pure U actually had a lower radioactivity than natural U compounds. To investigate this. Curie synthesized one of these compounds from pure reagents and found that the synthetic compound had a lower radioactivity than the identical natural example. This led her to believe that there was an impurity in the natural compound which was more radioactive than U (Curie 1898). Since she had already tested all the other elements, this impurity seemed to be a new element. In fact, it turned out to be two new elements—polonium and radium— which the Curies were successfully able to isolate from pitchblende (Curie and Curie 1898 Curie et al. 1898). For radium, the presence of a new element was confirmed by the observation of new spectral lines not attributable to any other element. This caused a considerable stir and the curious new elements, together with their discoverers, achieved rapid public fame. The Curies were duly awarded the 1903 Nobel prize in Physics for studies into radiation phenomena, along with Becquerel for his discovery of spontaneous radioactivity. Marie Curie would, in 1911, also be awarded the Nobel prize in chemistry for her part in the discovery of Ra and Po. 

All five elements in the oxygen group have six electrons in their outer orbits. They are all oxidizers (they accept electrons), but they are not all alike. They range from a nonmetal gas (oxygen) to a nonmetal solid (sulfur) to a nonmetallic semiconductor (selenium) to a semimetal (tellurium) and finally to a radioactive metal (polonium). 

They knew there must be another radioactive element in the pitchblende after the uranium was removed. Marie Curie painstakingly processed a ton of pitchblende to recover only a small amount of uranium. Even so, there was still something radioactive in all that processed pitchblende. As it turned out, there were two radioactive elements that she was able to isolate. One was radium, and the other polonium. They were identified by using piezoelectricity, discovered by her husband Pierre Curie, which could measure the strength of radiation given off by the radioactive elements with which Marie Curie was working. 

Radon-222 also undergoes radioactive decay and has a radioactive half-life of 3.8 days. Radon-220 and -219 have half-lives measured in seconds and are not nearly as abundant as Radon-222. Thus the discussion of radon health effects here centers on Radon-222. Radon-222 decays into radon daughters or progeny, which are radioactive elements. Two of these (polonium-218 and polonium-214) emit alpha particles (high-energy, high-mass particles, each consisting of two protons and... 

These two kinds of lead are now known to be isotopes, or inseparable elements which belong in the same space in the periodic table and yet differ in atomic weight and in radioactive properties. According to Frederick Soddy, the first clear recognition of isotopes as chemically inseparable substances was that of H. N. McCoy and W. H. Ross in 1907 (75,107). Strictly speaking, the science of radioactivity has revealed only five naturally occurring new elements with distinctive physical and chemical properties polonium, thoron, radium, actinium, and uranium X2. All the other natural radioactive elements share previously occupied places in the periodic table. 

Pierre and Marie Curie called Becquerel s radiation radioactivity . They found that another heavy element, thorium, was also radioactive, and deduced that natural uranium ore (pitchblende) contained other radioactive elements, which they called polonium (after Marie s native country) and radium (because it glowed). After two years of sifting through tonnes of uranium ore, they isolated salts of these new elements. The work left both the Curies with hands badly scarred from radiation bums, and it no doubt hastened Marie s death from leukaemia in 1934. Pierre might have met the same fate had he not been tragically killed in a road accident in 1906. 

The concept of close packing is particularly useful in describing the crystal structures of metals, most of which fall into one of three classes hexagonal close packed, cubic close packed (i.e., fee), and body-centered cubic (bcc). The bcc unit cell is shown in Fig. 4.8 its structure is not close packed. The stablest structures of metals under ambient conditions are summarized in Table 4.1. Notable omissions from Table 4.1, such as aluminum, tin, and manganese, reflect structures that are not so conveniently classified. The artificially produced radioactive element americium is interesting in that the close-packed sequence is ABAC..., while one form of polonium has... 

The production of artificially produced radioactive elements dales back to the early work of Rutherford in 1919 when it was found that alpha particles reacted with nitrogen atoms to yield protons and oxygen atoms. Curie and Joliot found (1933) that when boron, magnesium, or aluminum were bombarded with alpha particles from polonium, the elements would emit neutrons, protons, and positrons. They also found that upon cessation... 

Radium is chemically similar to barium it displays a characteristic optical spectrum its salts exhibit phosphorescence in the dark, a continual evolution of heat taking place sufficient in amount to raise the temperature of 100 times its own weight of water 1°C every hour and many remarkable physical and physiological changes have been produced. Radium shows radioactivity a million times greater than an equal weight of uranium and. unlike polonium, suffers no measurable loss of radioactivity over a short period of time (its half life is 1620 years). From solutions of radium salts, there is separable a radioactive gas radium emanation, radon, which is a chemically ineit gas similai to xenon and disintegrates with a half life of 3.82 days, with the simultaneous formation of another radioactive element, Radium A (polonium-218). 

Rutherford s work involved the use of alpha (a) particles, a type of emission previously observed to be given off by a number of naturally occurring radioactive elements, including radium, polonium, and radon. Rutherford knew that alpha particles are about 7000 times more massive than electrons and that they have a positive charge that is twice the magnitude of the charge on an electron, but opposite in sign. 

The group 6A elements are oxygen, sulfur, selenium, tellurium, and polonium. As shown in Table 19.7, their properties exhibit the usual periodic trends. Both oxygen and sulfur are typical nonmetals. Selenium and tellurium are primarily non-metallic in character, though the most stable allotrope of selenium, gray selenium, is a lustrous semiconducting solid. Tellurium is also a semiconductor and is usually classified as a semimetal. Polonium, a radioactive element that occurs in trace amounts in uranium ores, is a silvery white metal. 

At the end of long and hard days, they isolated a new element. From pitchblende , an uranium ore, they obtained a new element which radiates rays similar to uranium. They named this new element polonium to honor the memory of Poland, Marie Curie s homeland. This discovery led to the discovery of radium which made the Curies famous. With the discoveries of these new radioactive elements, the number of such elements reached four. They were uranium, thorium, polonium and radium. 

The non-radioactive element, gO was first produced artificially by Ernest Rutherford in 1919. In order to achieve this, he bombarded stable nucleus with a-particles emitted by radium and polonium. 

Marie Curie discovered the element polonium, Po, in 1898. She named polonium after Poland, her homeland. Curie won two Nobel Prizes, one in Physics (1903) for sharing in the discovery of radioactivity, and one in Chemistry (1911) for the discovery of radium, which has been used to treat cancer. Radium-226 undergoes alpha decay to yield radon-222. 

It is named after the Nobel prize winner Madame Curie, who discovered many properties of radioactive elements. She also discovered an element which she called polonium after the country in which she was born. 

In spite of all the new approaches which illuminated the outer regions of the atom, the center or nucleus of the atom continued to remain a bundle of uncertainties. Something of the composition of the nuclei of a few elements was already known. This information came from a study of the spontaneous disintegration of radium and other radioactive elements, such as thorium, polonium, uranium, and radon. These elements break down of their own accord into simpler elements. Soon after the Curies discovery of radium, Rutherford and Frederick Soddy, his student and collaborator, had found that the spontaneous breaking down of radium resulted in the emission of three types of rays and particles. Radium ejected alpha particles (ionized helium atoms), beta particles (electrons), and gamma rays (similar to X-rays). In radioactive elements, at least, it was believed that the nucleus contained electrons, protons, and electrified helium particles. 

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