Fission and Fusion of Atomic Nuclei

Albert Einstein s most famous equation relates mass and energy. 

The energy released when an atom s nucleons bind together is called binding energy. The greater the binding energy, the more stable a nucleus is. Elements with a mass number near 60 are the most stable. 

The mass of a nucleus is actually less than the sum of the masses of the nucleons. This difference is called the mass defect. The missing mass in the nucleus provides the energy that holds a nucleus together. 

One fission reaction can lead to more fission reactions, a process called a chain reaction. A chain reaction can occur only if the starting material has enough mass to sustain a chain reaction this amount is called critical mass. With a subcrifical mass, the chain reaction stops or never begins. With a supercritical mass, the chain reaction accelerates and can lead to a violent explosion. 

The Sun is powered by the fusion of hydrogen atoms into helium atoms. When the Sun has exhausted its hydrogen supply, it could fuse helium-4, forming carbon-12. Write a balanced nuclear equation for this process. 

In your study of chemical reactions, you learned that mass is conserved. For most practical situations this is true—but, in the strictest sense, it is not. It has been discovered that energy and mass can be converted into each other. Mass and energy are related by Albert Einstein s most famous equation. 

In this equation, AT stands for change in energy (joules). Am for change in mass (kg), and c for the speed of light (3.00 X 10 m/s). This equation is of major importance for all chemical and nuclear reactions, as it means a loss or gain in mass accompanies any reaction that produces or consumes energy. 

The figure shows the nuclear fission of uranium-235. When bombarded with a neutron, uranium-235 forms unstable uranium-236, which then splits into two lighter nuclei and additional neutrons. The splitting (fission) of the nucleus is accompanied by a large release of energy. 

This figure illustrates the ongoing reactions characteristic of a nuclear fission chain reaction. 

The amount of fissionable matter present determines whether a nuclear chain reaction can be sustained. In a subcritical mass, the chain reaction stops because neutrons escape the sample before causing sufficient fissions to sustain the reaction, o In a supercritical mass, the chain reaction accelerates as neutrons cause more and more fissions to occur. 

---

Important chain reactions, which under some conditions lead to explosion, are the fission and fusion of atomic nuclei (Section 20-22),... 

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. 

Fission and fusion differ from radioactive decay in that they generally require the nucleus of a parent atom to interact with an outside pcirticle (some manmade isotopes have been known to undergo fission without bombcirdment — Fe-256, for example). Because the forces that hold atomic nuclei together cire ridiculously powerful, the energy involved in splitting or joining two nuclei is tremendous. Here cire the differences between fusion and fission ... 

There are two types of nuclear reactions fission and fusion. In a fusion reaction, the nuclei of two atoms "fuse" to make a larger nucleus. Since the number of protons is different, this creates a different element. 

Fusion, the joining of light nuclei to form heavier nuclei, is favorable for the very light atoms. In both fission and fusion, the energy liberated is equivalent to the loss of mass that accompanies the reactions. Much greater amounts of energy per unit mass of reacting atoms are produced in fusion than in fission. 

A vast amount of energy is released when heavy atomic nuclei split—the nuclear fission process—and when small atomic nuclei combine to make heavier nuclei—the fusion process. In 1938, Otto Hahn, Fritz Strassman, Lise Meitner, and Otto Frisch discovered that ggU is fissionable by neutrons (Figure 13.8). In less than a decade, this discovery led to two important applications of this energy release accompanying fission—the atomic bomb and nuclear power plants. 

Neutrons are neutral particles present in all atomic nuclei except those of ordinary hydrogen. Neutrons are required to initiate the fission process, which in turn produces a large number of neutrons. Neutrons are produced by both fission and fusion reactions in nuclear (atomic) explosions. Neutrons and gamma radiation produced the direct instant radiation casualties in Hiroshima and Nagasaki. 

An entirely different kind of chemistry sub-discipline is nuclear chemistry. It deals with chemicals all right, but its concern is quite different from all the sub-disciplines mentioned above. It studies the nuclei of atoms in chemicals. The nuclei obey quite different kinds of rules than the ordinary chemicals do, as the next Chap. (19) explains. It treats the radioactivity, nucleosynthesis (how elements are produced), nuclear fission and fusion, and extends to cosmochemistry. 

In this chapter we consider the structure of the nucleus. We shall examine in some detail the nature of the bond (force) holding nucleons together in nuclei, and the properties of the nuclei such as radioactivity, artificial transmutations, fission, and fusion. Where possible, this discussion is in terms of principles previously applied to the properties of atoms and molecules. 

The fourth method of generation of energy is nuclear. Nuclear energy may be generated by die fission (splitting) of the atoms of certain elements and by the fusion (or joining together) of the nuclei of certain elements. 

---