The Nucleus and Radioactivity

Our journey into the center of the atom begins with a brief review. You learned in Chapter 2 that the protons and neutrons in each atom are found in a tiny, central nucleus that measures about 1/100,000 the diameter of the atom itself You also learned that the atoms of each element are not necessarily identical they can differ with respect to the number of neutrons in their nuclei. When an element has two or more species of atoms, each with the same number of protons but a different number of neutrons, the different species are called isotopes. Different isotopes of the same element have the same atomic number, but they have a different mass number, which is the sum of the numbers of protons and neutrons in the nucleus. In the context of nuclear science, protons and neutrons are called nucleons, because they reside in the nucleus. The atom s mass number is often called the nucleon number, and a particular type of nucleus, characterized by a specific atomic number and nucleon number, is called a nuclide. Nuclides are represented in chemical notation by a subscript atomic number (Z) and superscript nucleon number (A) on the left side of the element s symbol (X)  

For example, the most abundant nuclide of uranium has 92 protons and 146 

2 provide practice in writing and interpreting nuclide symbols. 

A nuclide that has 26 protons and 33 neutrons is used to study blood chemistry. Write its nuclide symbol in the form of 2X. Write two other ways to represent this nuclide. 

Because this nuclide has 26 protons, its atomic number, Z, is 26, identifying the element as iron, Fe. This nuclide of iron has 59 total nucleons (26 protons + 33 neutrons), so its nucleon number. A, is 59. 

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Madame Marie Curie, a Nobel Prize winning contributor to our understanding of the nucleus and radioactivity. 

Gamma ray The shortest wavelength and highest energy type of all electromagnetic radiation. It originates in the nucleus of radioactive isotopes along with alpha particle, beta particle, or neutron emissions. 

The most common types of radiation emitted by radioactive nuclei are a particles (the nuclei of helium atoms), /3 particles (fast electrons ejected from the nucleus), and 7 rays (high-frequency electromagnetic radiation). 

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. 

In essence, NAA involves converting some atoms of the elements within a sample into artificial radioactive isotopes by irradiation with neutrons. The radioactive isotopes so formed then decay to form stable isotopes at a rate which depends on their half-life. Measurement of the decay allows the identification of the nature and concentration of the original elements in the sample. If analysis is to be quantitative, a series of standard specimens which resemble the composition of the archaeological artifact as closely as possible are required. NAA differs from other spectroscopic methods considered in earlier chapters because it involves reorganization of the nucleus, and subsequent changes between energy levels within the nucleus, rather than between the electronic energy levels. 

What do we mean by a chemical element A chemical element is matter, all of whose atoms are alike in having the same positive charge on the nucleus and the same number of extra-nuclear electrons. As we shall see in the following elemental review, the origin of the chemical elements show a wide diversity with some of these elements having an origin in antiquity, other elements having been discovered within the past few hundred years and still others have been synthesized within the past fifty years via nuclear reactions on heavy elements since these other elements are unstable and radioactive and do not exist in nature. 

There are three significant possible effects when radiation interacts with matter (5,6). First, the radiation can interact with the nucleus and induce radioactivity as in the case of neutrons. Second, displacement of atoms can occur. This has happened in a number of uranium- and thorium-containing minerals over geological periods. The outstanding example is zircon, which can contain over 10% Th and 2% U. The internal bombardment from these materials and their decay products over geological periods produces low or metamict zircon, where the disorder gives an amorphous state having a low density. 

Sample Problem 1.2 Because of the conservation of the number of nucleons in the nucleus and conservation of charge during radioactive decay (Table 1.1),... 

Rutherford gave an estimate of the dimensions of the nucleus and then discussed the passage of (3 and a particles through matter, the transmutation of nitrogen into oxygen he had observed 2 years before,22 the existence of isotopes of both radioactive and stable elements, their separation, and finally the structure of nuclei for which a reduction of the mass had been established with respect to the sum of the masses of their constituents, which were assumed to be protons and electrons, in appropriate number. 

In eukaryotic cells the nuclear membrane separates the processes of RNA and protein synthesis. This can be demonstrated with radioactive substrates that are precursors of RNA and protein. Immediately after exposure of cells to labeled precursors, the RNA label becomes fixed in the nucleus, and the protein label becomes fixed in the cytoplasm. Eventually most of the labeled RNA becomes transferred to the cytoplasm, and a fraction of the labeled protein becomes transferred to the nucleus. 

Dalton s atoms were not perceived as possessing structure, but the discovery of electrons, and the distinctive phenomenon of radioactivity, inevitably generated interest in the way atoms were put together. Clearly, this had a bearing on the periodic table as any proposed structure must explain atomic weights.24"28 The Aufbau Principle, that each element possessed one more proton in the nucleus and one more electron in the outer shell than the preceding element, effectively systematized the periodic table29-30... 

Autoradiographic analysis is the only method which can determine the proportion of cells incorporating a radioactive precursor and the site of that incorporation. Thus, tritiated thymidine is incorporated into DNA in the nuclei of those cells in S-phase and tritiated hypoxanthine appears first in the nucleus and later in the cytoplasm of cells with HPRT but not in mutants lacking the enzyme (see 13.2). 

Permanency of Nucleus. The force which must be overcome in forcing protons together against their enormous electrostatic repulsion into the narrow confines of the nucleus is beyond our comprehension. No adequate theory has been devised to explain the stability of the nucleus yet the fact of the permanency of the nucleus exists. The identity of the element depends on the permanency of the nucleus, and our well-known law of the conservation of the elements expresses the almost absolute permanency of the nuclei of the common elements. Only with the radioactive elements is the nucleus subject to change, and this change is in most ways entirely unaffected by any physical or chemical forces which scientists are able to bring to bear. 

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. 

Notice that Tve written each atom or particle with a new notation. The subscript in front of each symbol gives the atomic number (the number of protons in the nucleus) and the superscript in front of each symbol gives the atomic mass (the total number of protons and neutrons in the nucleus). This is common notation in radioactive decay, so you can keep track of the number of protons and neutrons. Also notice that because the radon lost a couple of protons, it transformed into a new atom—polonium. The symbol for the helium nucleus is written with 2+ as a superscript because it has a charge of +2. Remember... 

Rutherford was one of the great scientific figures of the 20th century. He made a number of important discoveries about the structure of atoms and about radioactivity. For example, he found that an atom consists of two distinct parts, the nucleus and the electrons. He also discovered one form of radiation given off by radioactive materials alpha particles. Alpha particles, he found, are simply helium atoms without their electrons. 

The radionuclide at the beginning of the decay sequence is referred to as the parent, and the radionuclide produced by the decay is referred to as the daughter, which may be stable or radioactive. There are five types of radioactive decay, distinguished according to the nature of the primary radiation event. A radioactive nucleus may decay by more than one riKthod. The dominant method at any given time depends on such factors as the size of the nucleus and the balance of protons and neutrons. The types of decay described below are in order of how commonly they are u.sed in current diagnostic nuclear medicine practice ... 

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