Explanation:In order to understand nuclear reactions, we must review and develop some ideas introduced in Section 2.3. First, recall that two subatomic particles reside in the nucleus, the proton and the neutron. We will refer to these particles as nucleons. Recall also that all atoms of a given element have the same number of protons; this number is known as the element's atomic number. However, the atoms of a given element can have different numbers of neutrons and therefore different mass numbers; the mass number is the total number of nucleons in the nucleus. Atoms with the same atomic number but different mass numbers are known as isotopes.
The different isotopes of an element are distinguished by their mass numbers. For example, the three naturally occurring isotopes of uranium are identified as uranium-233, uranium-235, and uranium-238, where the numerical suffixes represent the mass numbers. These isotopes are also labeled, using chemical symbols, as U, U, and U. The superscript is the mass number; the subscript is the atomic number.
Different isotopes have different natural abundances. For example, 99.3 percent of naturally occurring uranium is uranium-238, 0.7 percent is uranium-235, and only a trace is uranium-233. Different nuclei also have different stabilities. Indeed, the nuclear properties of an atom depend on the number of protons and neutrons in its nucleus. Recall that the term nuclide applies to a nucleus with a specified number of protons and neutrons. (For more information, see Section 2.3) Nuclei that are radioactive are called radionuclides, and atoms containing these nuclei are called radioisotopes.
Nuclear Equations
The vast majority of nuclei found in nature are stable and remain intact indefinitely. Radionuclides, however, are unstable and spontaneously emit particles and electromagnetic radiation. Emission of radiation is one of the ways in which an unstable nucleus is transformed into a more stable one with less energy. The emitted radiation is the carrier of the excess energy. For example, uranium-238 is radioactive, undergoing a nuclear reaction in which helium-4 nuclei are spontaneously emitted. The helium-4 particles are known as alpha particles, and a stream of these particles is called alpha radiation. When a uranium-238 nucleus loses an alpha particle, the remaining fragment has an atomic number of 90 and a mass number of 234. It is therefore a thorium-234 nucleus. We represent this reaction by the following nuclear equation:
U Th + He
