![]() In this case, there are 94 electrons before and after the decay. The mass of the electrons is the same before and after α decay, and so their masses subtract out when finding Δ m. Note that the masses given in Appendix A are atomic masses of neutral atoms, including their electrons. The question of why the products have less mass will be discussed in Binding Energy. This decay is spontaneous and releases energy, because the products have less mass than the parent nucleus. The 235U nucleus can be left in an excited state to later emit photons ( γ rays). The energy carried away by the recoil of the 235U nucleus is much smaller in order to conserve momentum. Most of this energy becomes kinetic energy of the α particle (or 4He nucleus), which moves away at high speed. The energy released in this α decay is in the MeV range, about 10 6 times as great as typical chemical reaction energies, consistent with many previous discussions. Now we can find E by entering Δ m into the equation: E = (Δ m) c 2 = (0.005631 u) c 2. The decay equations for these two nuclides are Another nuclide that undergoes α decay is 239Pu. ![]() ![]() One example of α decay is shown in Figure 1 for 238U. In alpha decay, a 4He nucleus simply breaks away from the parent nucleus, leaving a daughter with two fewer protons and two fewer neutrons than the parent (see Figure 2). ![]() No γ decays are shown in the figure, because they do not produce a daughter that differs from the parent. Beta decay is a little more subtle, as we shall see. The daughters of β decay have one less neutron and one more proton than their parent. This seems reasonable, since we know that α decay is the emission of a 4He nucleus, which has two protons and two neutrons. Note that the daughters of α decay shown in Figure 1 always have two fewer protons and two fewer neutrons than the parent. A stable isotope of lead is the end product of the series. You can see why radium and polonium are found in uranium ore. Note that some nuclides decay by more than one mode. The type of decay for each member of the series is shown, as well as the half-lives. Nuclides are graphed in the same manner as in the chart of nuclides. The decay series produced by 238U, the most common uranium isotope. The 238U decay series ends with 206Pb, a stable isotope of lead.įigure 1. The decay of radon and its daughters produces internal damage. Since radon is a noble gas, it emanates from materials, such as soil, containing even trace amounts of 238U and can be inhaled. Radon gas is also produced ( 222Rn in the series), an increasingly recognized naturally occurring hazard. The decay series that starts from 238U is of particular interest, since it produces the radioactive isotopes 226Ra and 210Po, which the Curies first discovered (see Figure 1). Others, such as 238U, decay to another unstable nuclide, resulting in a decay series in which each subsequent nuclide decays until a stable nuclide is finally produced. For example, 60Co is unstable and decays directly to 60Ni, which is stable. Some radioactive nuclides decay in a single step to a stable nucleus. We call the original nuclide the parent and its decay products the daughters. Unstable nuclides decay (that is, they are radioactive), eventually producing a stable nuclide after many decays. Some nuclides are stable, apparently living forever. In this section, we explore the major modes of nuclear decay and, like those who first explored them, we will discover evidence of previously unknown particles and conservation laws. Nuclear decay gave the first indication of the connection between mass and energy, and it revealed the existence of two of the four basic forces in nature. Nuclear decay has provided an amazing window into the realm of the very small. Calculate the energy emitted during nuclear decay.By the end of this section, you will be able to:
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