The forces that bind protons and neutrons together in the nucleus usually are strong. However, in some isotopes, the nuclei are unstable because the forces binding protons and neutrons together are not strong enough. As a result, the nuclei spontaneously break apart (decay), a process called radioactivity. What happens when unstable nuclei break apart? Three common types of radioactive decay are illustrated in Figure 11.15 and are summarized as follows: Alpha (α) particles may be emitted from the nucleus. An alpha particle consists of two protons and two neutrons. Consequently, the emission of an alpha particle means that the mass number of the isotope is reduced by four and the atomic number is decreased by two.When a beta (β) particle, or electron, is given off from a nucleus, the mass number remains unchanged because electrons have practically no mass. However, because the electron has come from a neutron (remember, a neutron is a combination of a proton and an electron), the nucleus contains one more proton than before. Therefore, the atomic number increases by one.Sometimes an electron is captured by the nucleus. The electron combines with a proton and forms an additional neutron. As in the last example, the mass number remains unchanged. However, as the nucleus now contains one less proton, the atomic number decreases by one.- An unstable(radioactive) isotope of an element is called the parent. The isotopes resulting from the decay of the parent are the daughter products. - Figure 11.16 provides an example of radioactive decay. - Here it can be seen that when the radioactive parent, uranium-238(atomic number 92, mass number 238), decays, it follows a number of steps, emitting eight alpha particles and six beta particles before finally becoming the stable daughter product lead-206 (atomic number 82, mass number 206) - Certainly among the most important results of the discovery of radioactivity is that it provided a reliable means of calculating the ages of rocks and minerals that contain particular radioactive isotopes, a procedure called radiometric dating.- Why is radiometric dating reliable? Because the rates of decay for many isotopes have been precisely measured and do not vary under the physical conditions that exist in Earth's outer layers. - Therefore, each radioactive isotope used for dating has been decaying at a fixed rate since the formation of the rocks in which it occurs, and the products of S decay have been accumulating at a ponding rate. - For example, when uranium is into a mineral that crystallizes from magma, there is no lead(the stable daughter product) from previous decay.- The radiometric 'clock" starts at this point. - The radiometric 'clock" - As the uranium in this newly formed mineral disintegrates, atoms of the daughter product are trapped, and measurable amounts of lead eventually accumulate