The Discovery of Radioactivity

Phosphorescent minerals re-emit light after they have been exposed to light. 

As you saw in Section 3.2, identification of alpha particles as helium nuclei ( He) and beta particles as electrons played a significant role in our understanding of the structure of the atom. 

Before long it was recognized that the radiation from elements like uranium and radium consisted of three types, now known as alpha particles, beta particles, and gamma rays. 

In 1899, Ernest Rutherford found that alpha particles could be stopped by thin pieces of paper and had a range of only about 2.5 cm to 8.5 cm in air, whereas beta particles were capable of penetrating far greater distances in air. 

From the deflections, he determined that some of the particles were positively charged, some were negatively charged, and some were not charged at all. He discovered that the positively charged particles were much more massive than the others, and when combined with electrons, they formed helium atoms. He called these particles alpha particles (a- particles), and 

The remainder of this chapter will be devoted to the process of radioactive decay. 

Becquerel later retracted his results, however, when he discovered that a photographic plate with the same crystals showed a dark exposure spot even when the plate and the crystals were stored in a drawer and not exposed to sunlight. Becquerel realized 

The AP test requires you to know about nuclear equations, half-lives, radioactivity, and chemical applications of nuclear properties. This chapter begins with a brief review of the history of the nucleus and how we came to know about it and then moves into the required topics. 

In 1896, radioactivity was discovered. With that discovery came the realization that atoms are not immutable (unalterable), but that, in fact, they can be transmuted (changed) into atoms of dilferent elements. To change an atom of one element into an atom of a different element requires that the number of protons in the atom be changed. Such a transmutation is a nuclear reaction and is fundamentally different from the ordinary chemical reactions that have been discussed thus far. Ordinary chemical reactions are all about electrons— there are no changes in the numbers of protons or neutrons. On the other hand, nuclear reactions are all about protons and neutrons—the electrons usually are not part of nuclear reactions, although their number can change to remain equal to the number of protons in an electrically neutral atom. 

I querel later retracted his results, however, when he discovered that a photographic plate with the same crystals showed a bright exposure spot even when the plate and the crystals were stored in a dark drawer and not exposed to simlight. Becquerel realized that the crystals themsdves were constantly emitting something (independent of whether or not they phosphoresced) that exposed the photographic plate. Becquerel concluded that it was the uranium within the crystals that was the soiuce of the emissions, and he called the emissions uranic rays. 

Soon after Becquerel s discovery, a yoimg graduate student named Marie Sklodowska Ciuie (1867-1934), one of the first women in France to attempt doctoral work, decided to pursue the study of manic rays for her doctoral thesis. Her first task was to determine whether any other substances besides uranium (the 

Element 96 (curium) is named in honor of Marie Curie and her contributions to our understanding of radioactivity. 

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By this time, the Periodic Table of elements was well developed, although it was considered a function of the atomic mass rather than atomic number. Before the discovery of radioactivity, it had been estabUshed that each natural element had a unique mass thus it was assumed that each element was made up of only one type of atom. Some of the radioactivities found in both the uranium and thorium decays had similar chemical properties, but because these had different half-Hves it was assumed that there were different elements. It became clear, however, that if all the different radioactivities from uranium and thorium were separate elements, there would be too many to fit into the Periodic Table. 

The discovery and detailed investigations of the phenomenon of fluorescence is generally considered the main contribution of Edmond Becquerel. It had the further impact of leading later to the discovery of radioactivity by his son Henri, as Henri continued th ese studies, including among the substances examined salts of uranium. 

The development of particle accelerators grew out of the discovery of radioactivity in uranium by Henri Becquerel in Paris in 1896. Some years later, due to the work of Ernest Rutherford and others, it was found that the radioactivity discovered by Becquerel was the emission o particles with kinetic energies o several MeV from uranium nuclei. Research using the emitted particles began shortly thereafter. It was soon realized that if scientists were to learn more about the properties of subatomic particles, they had to be accelerated to energies greater than those attained in natural radioactivity. 

In 1903, the Curies received the Nobel Prize in physics (with Becquerel) for the discovery of radioactivity. Three years later, Pierre Curie died at the age of 46, the victim of a tragic accident. Fie stepped from behind a carriage in a busy Paris street and was run down by a horse-driven truck. That same year, Marie became the first woman instructor at the Sorbonne. In 1911, she won the Nobel Prize in chemistry for the discovery of radium and polonium, thereby becoming the first person to win two Nobel Prizes. 

One hundred years after the discovery of radioactivity and fifty years after the dawn of the nuclear age, society continues to debate the benefits and costs of nuclear technology. Understanding nuclear transformations and the properties of radioactivity is necessary for intelligent discussions of the nuclear dilemma. In this chapter, we explore the nucleus and the nuclear processes that it undergoes. We describe the factors that make nuclei stable or unstable, the various types of nuclear reactions that can occur, and the effects and applications of radioactivity. 

Chemists were not able to use their methods to determine the structure of the atom. The discovery of radioactivity by Henri Becquerel and the work of Marie and Pierre Curie showed, however, that heavy elements were not stable. The earlier postulate of their indivisibility could no longer be maintained. In 1906 Ernest Rutherford made the next horrorif-ic revelation his scattering experiments showed that the atom was almost empty. A tiny nuclear mass was circled by electrons at a large distance. For comparison, if the nucleus were the size of a cherry pit and were placed in the center of a football field, the electrons would be circulating in the back rows of the stadium. If the nucleus were the size of a football, the first electrons would be circling it at a distance of one kilometer. Between them would be absolute emptiness. 

FIGURE 88 Dating methods. Shortly after the discovery of radioactivity, at the beginning of the twentieth century, it was found that the decay of radioactive elements could be used to keep track of time. Many of the dating techniques developed since then are, therefore, based on radioactive decay phenomena, but others, such as the hydration of obsidian, amino acid racemization, and dendrochronology, are based on other physical, chemical, or biological phenomena. 

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. 

But until the discovery of radioactive transmutation by Rutherford and Soddy, many outside of occult circles were content to view alchemy as simply... 

Section 1.2 deals with the time period from Dalton to the discovery of isotopes by Soddy and Fajans. Much of the discussion elaborates on the type of material found in introductory chemistry texts. It ends with the discovery of radioactivity by Becquerel and the developments which quickly followed. Section 1.3 starts with the discovery of the concept of isotopes in the early years of the twentieth century and ends with the invention of the mass spectrograph in 1922 by Aston. The literature relating to the work leading up to the 1913 papers by Soddy and Fajans is well and... 

Atomic (or Nuclear) Energy Atomic (or Nuclear) Reactions Atomic (or Nuclear) Explosions. In chemical reactions the atomic nuclei maintain their charges, masses, and individual identities. These all change in aromic or nuclear reactions, first revealed in the discovery of radioactivity by Becquerel in 1895 and of radium by the Curies in 1898. 

Romer, A. The Discovery of Radioactivity and Transmutation, Dover, New York, 1964. 

Smithells, initially, expressed his excitement about the state of chemistry. Though the discovery of radioactivity did mark a new epoch in the history of chemistry, and radium was in a way an embarrassment, since it was elementary and it also broke into elementary substances, there was not enough evidence to warrant any unsettlement of the scientific articles of the chemists faith. ... 

The development of nuclear physics began with the discovery of radioactivity by the French scientist Henri Becquerel in 1896, and associated fundamental works by Pierre and Marie Curie who all received the Nobel Prize in physics in 1903. 

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. 

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