Chemistry with Atomic Nuclei

J Ju elements in the periodic table exist in unstable versions called radioisotopes (see Chapter 3 for details). These radioisotopes decay into other (usually more stable) elements in a process called radioactive decay. Because the stability of these radioisotopes depends on the composition of their nuclei, radioactivity is considered a form of nuclear chemistry. Unsurprisingly, nuclear chemistry deals with nuclei and nuclear processes. Nuclear fusion, which fuels the sun, and nuclear fission, which fuels a nuclear bomb, are examples of nuclear chemistry because they deal with the joining or splitting of atomic nuclei. In this chapter, you find out about nuclear decay, rates of decay called half-lives, and the processes of fusion and fission. 

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Chapter 20 Doing Chemistry with Atomic Nuclei 275... 

Sir Ernest Rutherford (1871-1937 Nobel Prize for chemistry 1908, which as a physicist he puzzled over) was a brilliant experimentalist endowed with an equal genius of being able to interpret the results. He recognized three types of radiation (alpha, beta, and gamma). He used scattering experiments with alpha radiation, which consists of helium nuclei, to prove that the atom is almost empty. The diameter of the atomic nucleus is about 10 000 times smaller than the atom itself. Furthermore, he proved that atoms are not indivisible and that in addition to protons, there must also be neutrons present in their nucleus. With Niels Bohr he developed the core-shell model of the atom. 

The Mossbauer effect as a spectroscopic method probes transitions within an atom s nucleus and therefore requires a nucleus with low-lying excited states. The effect has been observed for 43 elements. For applications in bioinorganic chemistry, the 57Fe nucleus has the greatest relevance and the focus will be exclusively on this nucleus here. Mossbauer spectroscopy requires (a) the emission of y rays from... 

Before beginning a discussion of nuclei and their properties, we need to understand the environment in which most nuclei exist, that is, in the center of atoms. In elementary chemistry, we learn that the atom is the smallest unit a chemical element can be divided into that retains its chemical properties. As we know from our study of chemistry, the radii of atoms are approximately 1-5 x 10-10 m, or 1 -5 A. At the center of each atom we find the nucleus, a small object (r 1-10 x 10-15 m) that contains almost all the mass of the atom (Fig. 1.1). The atomic nucleus contains Z protons, where Z is the atomic number of the element under study, Z being number of protons and is thus the number of positive charges in the nucleus. The chemistry of the element is controlled by Z in that all nuclei with the same Z will have similar chemical behavior. The nucleus also contains N neutrons, where N is the neutron number. Neutrons are uncharged particles with masses approximately equal to the mass of a proton ( 1 u). Each proton has a positive charge equal to that of an electron. The overall charge of a nucleus is +Z electronic charge units. 

The term atom and the names of its constituents—protons, neutrons, and electrons—have become almost household words during the last decade. The schematic sketch of the solar-system atom with a nucleus at the center and electrons revolving about it in circular or elliptical orbits has become as familiar as many of the commercial trademarks. This picture of the atom is that which is often presented to the student during his early chemistry courses while such a picture fits a number of important properties of atoms, it is no longer regarded as a good approximation. 

The idea of energy quantization weis brought into chemistry with the application of quantum theory to the electronic structure of atoms in 1913 by the Danish physicist Niels Bohr (1885-1962, 1922 Nobel laureate in Physics). At the time, Bohr was working in the laboratory of the New Zealand physidst Ernest Rutherford (1871-1937, 1909 Nobel laureate in Chemistry) in England, a short time after the nuclear structure for the atom had been established by Rutherford and his co-workers. Classical electromagnetic theory predicted that the electrons around the nucleus. 

The space electrons occupy around an atomic nucleus is described by atomic orbitals. The most common orbitals in organic chemistry are s-orbitals, spherical orbitals with the atomic nucleus located in the center, and dumbbell shaped p-orbitals in which the nucleus is between the lobes. 

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. 

Although much of quantum chemistry is insensitive to details of the nuclear structure, relativistic effects are generated in regions near the nucleus where, classically, electrons move fastest. The behaviour as r —0 depends on what model we choose to represent the atomic nucleus. We therefore start with some generalities. 

With the discovery of the neutron as a fundamental particle, many paradoxes of physics and chemistry were finally resolved, and new areas of research evolved. Prior to the discovery of the neutron as a fundamental particle, scientists generally believed that the nucleus was comprised of protons and nuclear electrons. However, one could not explain, for example, the spin of nuclei with that model. Now, at last, theory could predict the properties of the nucleus quite well. Also, since neutrons are not repelled by the charge on the atomic nucleus, they interact easily with nnclei. Nen-tron scattering enables the determination of crystal stmctnres by probing the positions of nuclei in a sample. Neutrons can also catalyze fission reactions, for example, the fission of uranium nuclei that led to the creation of nuclear power plants and the atomic bomb. 

While I am not a licensed lit-crit (literary critic), I think that a 1916 reviewer of another Lewis book had him properly pegged as a novelist The plot moves swiftly with the help of incredible coincidences and improbable ro-mances. Nevertheless, let us give the author very considerable credit as a knowledgeable and sophisticated observer of chemistry. He was amazingly well informed and current about the complex revolution in the understanding of the structure of the atomic nucleus that was very much in motion as he wrote White Lightning. Marvin reads of Henry G.J. Moseley s discovery of atomic numbers in 1914 12 unknown Moseley had found it—a sure way to determine the... 

The atomic weight of an element comes from the nucleus. Protons and neutrons in the nucleus each have a weight of 1 AMU. Atoms of elements have varying numbers of protons and neutrons. The total number of neutrons and protons in each atom equals the atomic weight. The atomic number of an element equals the number of protons in that element. The number of neutrons is determined by subtracting the number of protons from the atomic weight what remains is the number of neutrons. In chemistry, we are concerned more with the electrons orbiting the nucleus than the nucleus itself. We are particularly interested in the electrons in the outer shell of the atom. Chemical activity takes place between the outer-shell electrons of elements this chemical activity forms compounds. In radioactivity, the concern is with the nucleus, where radiation is emitted. Radioactivity will be discussed further in Chapter 8. 

Another area of general chemistry with which you should be familiar is the study of radioactivity, or nuclear chemistry. Specifically, nuclear chemistry deals with the properties of the nucleus of the atoms that s why it is called nuclear chemistry. 

Bohr learned about radiochemistry from de Hevesy. He began to see connections with his electron-theory work. His sudden burst of intuitions then was spectacular. He realized in the space of a few weeks that radioactive properties originated in the atomic nucleus but chemical properties depended primarily on the number and distribution of electrons. He realized—the idea was wild but happened to be true—that since the electrons determined the chemistry and the total positive charge of the nucleus determined the number of electrons, an element s position on the periodic table of the elements was exactly the nuclear charge (or atomic number ) hydrogen first with a nuclear charge of 1, then helium with a nuclear charge of 2 and so on up to uranium at 92. 

All science is based on a number of postulates. Quanmm mechanics has also elaborated a system of postulates that have been formulated to be as simple as possible and yet to be consistent with experimental results. Postulates are not supposed to be proved-their justification is efficiency. Quantum mechanics, the foundations of which date from 1925 and 1926, still represents the basic theory of phenomena within atoms and molecules. This is the domain of chemistry, biochemistry, and atomic and nuclear physics. Further progress (quantum electrodynamics, quantum field theory, and elementary particle theory) permitted deeper insights into the structure of the atomic nucleus but did not produce any fundamental revision of our understanding of atoms and molecules. Matter as described by non-relativistic quantum mechanics represents a system of electrons and nuclei, treated as pointlike particles with a definite mass and electric... 

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. 

From your study of general chemistry, you may already be familiar with the concept that an electron has a spin and that a spinning charge creates an associated magnetic field. In effect, an electron behaves as if it is a tiny bar magnet. An atomic nucleus that has an odd mass or an odd atomic number also has a spin and behaves as if it is a tiny bar magnet. Recall that when designating isotopes, a superscript represents the mass of the element. 

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