Atoms, nucleus radioactive

Becquerel presented the problem to Marie and Pierre Curie for further study. Their conclusion was that a nuclear reaction was taking place within the uranium atoms. Marie Curie named this spontaneous emission of radiation by an unstable atomic nucleus radioactivity. 

Nuclear chemistry is very much in the news today. In addition TO APPLICATIONS IN THE MANUFACTURE OF ATOMIC BOMBS, HYDROGEN BOMBS, AND NEUTRON BOMBS, EVEN THE PE.A.CEFUL USE OF NUCLEAR ENERGY HAS BECOME CONTROVERSIAL, IN PART BECAUSE OF SAFETY CONCERNS ABOUT NUCLEAR POWER PLANTS AND ALSO BECAUSE OF PROBLEMS WITH DISPOSAL OF RADIOACTIVE WASTES. IN THIS CHAPTER WE WILL STUDY NUCLEAR REACTIONS, THE STABILITY OF THE ATOMIC NUCLEUS, RADIOACTIVITY, AND THE EFFECTS OF RADIATION ON BIOLOGICAL SYSTEMS. 

This branch of chemistry began with the discovery of natural radioactivity by Antoine Becquerel and grew as a result of subsequent investigations by Pierre and Marie Curie and many others. Nuclear chemistry is very much in the news today. In addition to applications in the manufacture of atomic bombs, hydrogen bombs, and neutron bombs, even the peaceful use of nuclear energy has become controversial, in part because of safety concerns about nuclear power plants and also because of problems with radioactive waste disposal. In this chapter, we wiU study nuclear reactions, the stability of the atomic nucleus, radioactivity, and the effects of radiation on biological systems. 

The origin of the rays was initially a mystery, because the existence of the atomic nucleus was unknown at the time. However, in 1898, Ernest Rutherford took the first step to discover their origin when he identified three different types of radioactivity by observing the effect of electric fields on radioactive emissions (Fig. 17.4). Rutherford called the three types a (alpha), (3 (beta), and y (gamma) radiation. 

Two of these isotopes, carbon-12, the most abundant, and carbon-13 are stable. Carbon-14, on the other hand, is an unstable radioactive isotope, also known as radiocarbon, which decays by the beta decay process a beta particle is emitted from the decaying atomic nucleus and the carbon-14 atom is transformed into an isotope of another element, nitrogen-14, N-14 for short (chemical symbol 14N), the most common isotope of nitrogen ... 

Neutrons have no electrical charge and have nearly the same mass as a proton (a hydrogen atom nucleus). A neutron is hundreds of times larger than an electron, but one quarter the size of an alpha particle. The source of neutrons is primarily nuclear reactions, such as fission, but they are also produced from the decay of radioactive elements. Because of its size and lack of charge, the neutron is fairly difficult to stop, and has a relatively high penetrating power. 

Chemical forms with at least one radioactive atomic nucleus are radioactive substances. The capability of atomic nuclei to undergo spontaneous nuclear transformation is called radioactivity. Nuclear transformations are accompanied by emission of nuclear radiation (Severa and Bar 1991). The average number of nuclei that disintegrate per unit time (= activity) is directly proportional to the total number of radioactive nuclei. The time for 50% of the original nuclei to disintegrate (= half-life or Tb 1/2) is equal to In 2/decay constant for that element (Kiefer 1990). Radiations... 

The stability of the atomic nucleus depends upon a critical balance between the repulsive and attractive forces involving the protons and neutrons. For the lighter elements, a neutron to proton ratio (N P) of about 1 1 is required for the nucleus to be stable but with increasing atomic mass, the N P ratio for a stable nucleus rises to a value of approximately 1.5 1. A nucleus whose N P ratio differs significantly from these values will undergo a nuclear reaction in order to restore the ratio and the element is said to be radioactive. There is, however, a maximum size above which any nucleus is unstable and most elements with atomic numbers greater than 82 are radioactive. 

Scientists initially described radioactivity solely in terms of radiation. The idea of radioactive parf/c/esfirst appeared around the turn of the twentieth century. In 1909, Ernest Rutherford reported confidently that the alpha particle was, in fact, a helium nucleus, jHe, with a 2+ charge. Scientists still had not discovered the proton by this time, so the nature of the helium nucleus (or any other atomic nucleus) was still unknown. In 1919, the existence of the proton was confirmed experimentally by, appropriately enough, Rutherford himself. 

Today, physical chemistry has accomplished its great task of elucidating the microcosmos. The existence, properties and combinatory rules for atoms have been firmly established. The problem now is to work out where they came from. Their source clearly lies outside the Earth, for spontaneous (cold) fusion does not occur on our planet, whereas radioactive transmutation (breakup or decay), e.g. the decay of uranium to lead, is well known to nuclear geologists. The task of nuclear astrophysics is to determine where and how each species of atomic nucleus (or isotope) is produced beyond the confines of the Earth. 

Apart from these three facts, nuclear astrophysicists take pains to point out that the rate at which the luminosities of SNla events decline, once beyond the maximum, is commensurable with the decay of radioactive cobalt-56, son of nickel-56, atomic nucleus of noble lineage as we know. This is a common factor with gravitational collapse supernovas. SNla light curves are explained through the hypothesis that half a solar mass of nickel-56 is produced when one of these white dwarfs explodes. 

Radioactive elements are those which have a naturally unstable atomic nucleus. In the case of radium, two protons and two neutrons bonded together - an "alpha particle" -... 

In 1899 he identified two forms of radioactivity, which he called alpha and beta particles. As we saw earlier, he deduced that alpha particles are helium nuclei. Beta particles are electrons - but, strangely, they come from the atomic nucleus, which is supposed to be composed only of protons and neutrons. Before the discovery of the neutron this led Rutherford and others to believe that the nucleus contained some protons intimately bound to electrons, which neutralized their charge. This idea became redundant when Chadwick first detected the neutron in 1932 but in fact it contains a deeper truth, because beta-particle emission is caused by the transmutation ( decay ) of a neutron into a proton and an electron. 

Radioactivity Spontaneous decay or disintegration of an unstable atomic nucleus accompanied by the emission of radiation. 

The second reason the stabilizing effect of neutrons is limited is that any proton in the nucleus is attracted by the strong nuclear force only to adjacent protons but is electrically repelled by all other protons in the nucleus. As more and more protons are squeezed into the nucleus, the repulsive electric forces increase substantially. For example, each of the two protons in a helium nucleus feels the repulsive effect of the other. Each proton in a nucleus containing 84 protons, however, feels the repulsive effects of 83 protons The attractive nuclear force exerted by each neutron, however, extends only to its immediate neighbors. The size of the atomic nucleus is therefore limited. This in turn limits the number of possible elements in the periodic table. It is for this reason that all nuclei having more than 83 protons are radioactive. Also, the nuclei of the heaviest elements produced in the laboratory are so unstable (radioactive) that they exist for only fractions of a second. 

Radioactivity The tendency of some elements, such as uranium, to emit radiation as a result of changes in the atomic nucleus. 

Alpha particle A helium atom nucleus, which consists of two neutrons and two protons and is ejected by certain radioactive elements. 

Beta particle An electron ejected from an atomic nucleus during the radioactive decay of certain nuclei. 

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