21: Nuclear Chemistry
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Until now, you have studied chemical processes in which atoms share or transfer electrons to form new compounds, leaving the atomic nuclei largely unaffected. In this chapter, we examine some properties of the atomic nucleus and the changes that can occur in atomic nuclei. Nuclear reactions differ from other chemical processes in one critical way: in a nuclear reaction, the identities of the elements change. In addition, nuclear reactions are often accompanied by the release of enormous amounts of energy, as much as a billion times more than the energy released by chemical reactions. Moreover, the yields and rates of a nuclear reaction are generally unaffected by changes in temperature, pressure, or the presence of a catalyst.
We begin by examining the structure of the atomic nucleus and the factors that determine whether a particular nucleus is stable or decays spontaneously to another element. We then discuss the major kinds of nuclear decay reactions, as well as the properties and uses of the radiation emitted when nuclei decay. You will learn how radioactive emissions can be used to study the mechanisms of chemical reactions and biological processes and how to calculate the amount of energy released during a nuclear reaction. You will also discover why houses are tested for radon gas, how radiation is used to probe organs such as the brain, and how the energy from nuclear reactions can be harnessed to produce electricity. Last, we explore the nuclear chemistry that takes place in stars, and we describe the role that stars play in producing most of the elements in the universe.
- 21.1: Radioactivity
- Nuclei can undergo reactions that change their number of protons, number of neutrons, or energy state. Many different particles can be involved and the most common are protons, neutrons, positrons, alpha (α) particles, beta (β) particles (high-energy electrons), and gamma (γ) rays (which compose high-energy electromagnetic radiation). As with chemical reactions, nuclear reactions are always balanced. When a nuclear reaction occurs, the total mass (number) and the total charge remain unchanged.
- 21.2: Patterns of Nuclear Stability
- Protons and neutrons are called nucleons and a nuclide is an atom with a specific number nucleons. Unstable nuclei decay spontaneously are radioactive and its emissions are called radioactivity. Nuclei are bound by the strong nuclear force. Stable nuclei generally have even numbers of protons and neutrons with a ratio of at least 1. Nuclei that contain magic numbers of protons and neutrons are often especially stable including superheavy elements, with atomic numbers near 126.
- 21.3: Nuclear Transmutations
- Hydrogen and helium are the most abundant elements in the universe. Heavier elements are formed in the interior of stars via multiple neutron-capture events. Successive fusion reactions of helium nuclei at higher temperatures create elements with even numbers of protons and neutrons up to Mg and then up to Ca. Eventually, the elements up to Fe-56 and Ni-58 are formed by exchange processes at even higher temperatures. Heavier elements can only be made by the explosion of a supernova.
- 21.4: Rates of Radioactive Decay
- Unstable nuclei undergo spontaneous radioactive decay. The most common types of radioactivity are α decay, β decay, γ emission, positron emission, and electron capture. Nuclear reactions also often involve γ rays, and some nuclei decay by electron capture. Each of these modes of decay leads to the formation of a new stable nuclei sometimes via multiple decays before ending in a stable isotope. All nuclear decay processes follow first-order kinetics and each radioisotope has its own half-life.
- 21.5: Energy Changes in Nuclear Reactions
- Unlike a chemical reaction, a nuclear reaction results in a significant change in mass and an associated change of energy, as described by Einstein’s equation. Nuclear reactions are accompanied by large changes in energy, which result in detectable changes in mass. The experimentally determined mass of an atom is always less than the sum of the masses of the component particles (protons, neutrons, and electrons) by an amount called the mass defect that corresponds to the nuclear binding energy.
- 21.6: Nuclear Fission
- Many heavier elements with smaller binding energies per nucleon can decompose into more stable elements that have intermediate mass numbers and larger binding energies per nucleon. Sometimes neutrons are also produced. This decomposition is called fission, the breaking of a large nucleus into smaller pieces. The breaking is rather random with the formation of a large number of different products. Fission usually does not occur naturally, but is induced by bombardment with neutrons.
- 21.7: Nuclear Fusion
- The process of converting very light nuclei into heavier nuclei is also accompanied by the conversion of mass into large amounts of energy, a process called fusion. The principal source of energy in the sun is a net fusion reaction in which four hydrogen nuclei fuse and produce one helium nucleus and two positrons. This is a net reaction of a more complicated series of events.
- 21.8: Biological Effects of Radiation
- The effects of radiation on matter depend on the energy of the radiation. Nonionizing radiation is relatively low in energy, and the energy is transferred to matter in the form of heat. Ionizing radiation is relatively high in energy, and when it collides with an atom, it can completely remove an electron to form a positively charged ion that can damage biological tissues.
- 21.E: Exercises
- Problems and select solutions to chapter 21 of the Brown et al. textmap.
- 21.S: Nuclear Chemistry (Summary)
- Summary of Chapter 21 of the Brown et al. textmap.