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20 Radioactivity and Nuclear Chemistry Review Questions 20.1 Radioactivity is the emission of subatomic particles or high-energy electromagnetic radiation by the nuclei of certain atoms. Radioactivity was discovered in 1896 by the French scientist Antoine-Henri Becquerel (1852-1908). Becquerel placed crystals-composed of potassium uranyl sulfate, a com- pound known to phosphoresce-on top of a photographic plate wrapped in black cloth. The photo- graphic plate showed a bright exposure spot where the crystals had been. Becquerel, Marie Curie, and Pierre Curie received the Nobel Prize for the discovery of radioactivity. 20.3 A is the mass number (number of protons + neutrons), Z is the atomic number (number of protons), and X is the chemical symbol of the element. 20.5 An alpha particle has the same symbol as a helium nucleus, When an element emits an alpha particle, the number of protons in its nucleus decreases by two and the mass number decreases by four, transforming it into a different element. 20.7 Gamma rays are high-energy (short-wavelength) photons that have a symbol of A gamma ray has no charge and no mass. When a gamma-ray photon is emitted from a radioactive atom, it does not change the mass number or the atomic number of the element. Gamma rays, however, are usually emitted in conjunction with other types of radiation. 20.9 Electron capture occurs when a nucleus assimilates an electron from an inner orbital of its electron cloud. Like positron emission, the net effect of electron capture is the conversion of a proton into a neutron: + When an atom undergoes electron capture, its atomic number decreases by one because it has one less proton and its mass number is unchanged. 20.11 For the lighter elements, the N/Z ratio of stable isotopes is about one (equal numbers of neutrons and protons). However, beyond about Z = 20, the N/Z ratio of stable nuclei begins to get larger (reaching about 1.5). Above Z = 83, stable nuclei do not exist. If the N/Z ratio is too high, it indicates the pres- ence of too many neutrons and so the neutrons will be converted to protons via beta decay. If the N/Z ratio is too low, it indicates the presence of too many protons and therefore position emission or electron capture will be used to convert protons to neutrons. 20.13 (a) Film-badge dosimeters consist of photographic film held in a small case that is pinned to clothing and are standard for most people working with or near radioactive substances. These badges are collected and processed (or developed) regularly to monitor a person's exposure. The more exposed the film has become in a given period of time, the more radioactivity to which the person has been exposed. (b) A Geiger-Müller counter (commonly referred to as a Geiger counter) is an instrument that can detect radioactivity instantaneously. Particles emitted by radioactive nuclei pass through an argon-filled chamber. The energetic particles create a trail of ionized argon atoms. An applied high voltage between a wire within the chamber and the chamber itself causes these newly formed ions to produce an electrical signal that can be detected on a meter or turned into an audible click. Each click corresponds to a radioactive particle passing through the argon gas chamber. This clicking is the stereotypical sound most people associate with a radiation detector. 448 Copyright © 2017 Pearson Education, Inc.