Nuclides, unstable

Because of the long time scale involved in the s-process, unstable nuclides formed by (n.y) reactions have time to decay subsequently by decay (electron emission). The crucial factor in determining the relative abundance of elements... 

Figure 17.13 is a plot of mass number against atomic number for known nuclides. Stable nuclei are found in a band of stability surrounded by a sea of instability, the region of unstable nuclides that decay with the emission of radiation. For atomic numbers up to about 20, the stable nuclides have approximately equal numbers of neutrons and protons, and so A is close to 2Z. For higher atomic numbers, all known nuclides—both stable and unstable—have more neutrons than protons, and so A > 2Z. 

C22-0004. Write the symbols and determine Z, N, and A for the following nuclides (a) an oxygen nucleus with the same number of neutrons and protons (b) element 43 with 55 neutrons and (c) the unstable nuclide of hydrogen with the lowest mass. 

Unstable nuclides decompose spontaneously Into other, more stable nuclides. These decompositions are called nuclear decay, and unstable nuclides are called radioactive. Three features characterize nuclear decays the... 

There are five fundamental t T)es of nuclear decay process, as listed in Table 22-3. Figure 22-5 on the next page diagrams how nuclear decays affect N and Z. As the figure suggests, the decay process of any particular unstable nuclide depends on the reason for its instability. 

Radon-222 is an unstable nuclide that has been detected in the air of some homes. Its presence is a concern because of high health hazards associated with exposure to its radioactivity. Gaseous radon easily enters the lungs, and once it decays, the products are solids that remain embedded in lung tissue. Radon-222 transmutes to a stable nuclide by emitting a and P particles. The first four steps are a, a, P, p. Write this sequence of nuclear reactions and identify each product. 

A sample of any unstable nuclide undergoes nuclear decay continuously as its individual nuclei undergo reaction. All nuclear decays obey the first-order rate law Rate = C. This rate law can be treated mathematically to give Equation, which relates concentration, c, to time, t, for a first-order process (Cq is the concentration present at... 

One way to create unstable nuclides is by neutron capture. Because neutrons have no electrical charge, they readily penetrate any nucleus and may be captured as they pass through a nucleus. The sun emits neutrons, so a continuous stream of solar neutrons bathes the Earth s atmosphere. The most abundant nuclide in the atmosphere,... 

Almost every nuclide undergoes neutron capture if a source of neutrons is available. Unstable nuclides used in radiochemical applications are manufactured by neutron bombardment. A sample containing a suitable target nucleus is exposed to neutrons coming from a nuclear reactor (see Section 22-1). When a target nucleus captures a... 

Neutrons readily induce nuclear reactions, but they always produce nuclides on the high neutron-proton side of the belt of stability. Protons must be added to the nucleus to produce an unstable nuclide with a low neutron-proton ratio. Because protons have positive charges, this means that the bombarding particle must have a positive charge. Nuclear reactions with positively charged particles require projectile particles that possess enough kinetic energy to overcome the electrical repulsion between two positive particles. 

The Earth s age can be estimated by using the half-lives of unstable nuclides that are still present and the half-lives of those that are missing. Among those listed in Table 22-4. the shortest-lived naturally occurring nuclide is... 

Decay, Radioactive—Transformation of the nucleus of an unstable nuclide by spontaneous emission of charged particles and/or photons (see Disintegration). 

The numerical combination of protons and neutrons in most nuclides is such that the nucleus is quantum mechanically stable and the atom is said to be stable, i.e., not radioactive however, if there are too few or too many neutrons, the nucleus is unstable and the atom is said to be radioactive. Unstable nuclides undergo radioactive transformation, a process in which a neutron or proton converts into the other and a beta particle is emitted, or else an alpha particle is emitted. Each type of decay is typically accompanied by the emission of gamma rays. These unstable atoms are called radionuclides their emissions are called ionizing radiation and the whole property is called radioactivity. Transformation or decay results in the formation of new nuclides some of which may themselves be radionuclides, while others are stable nuclides. This series of transformations is called the decay chain of the radionuclide. The first radionuclide in the chain is called the parent the subsequent products of the transformation are called progeny, daughters, or decay products. 

Three primary problem areas exist in dating groundwater. These are (1) Formulation of realistic geochemical-hydrodynamic models needed to interpret data which are generated by field and laboratory measurements, (2) development of sensitive and accurate analytical methods needed to measure trace amounts of various stable and unstable nuclides, and (3) theoretical and field oriented studies to determine with greater accuracy the extent and distribution of the subsurface production of radionuclides which are commonly assumed to originate only in the atmosphere. 

A (3-particle is an electron. An unstable nuclide in (3-particle production creates an electron as it releases energy in the decay process. This electron is created from the decay process, rather than being present before the decay occurs. 

Figure 11.2 Enlarged portions of Segre chart of nuclides, showing s-process (upper chain) and r-process (lower chain). White boxes stable nuclides diagonally ruled boxes unstable nuclides crosshatched boxes highly unstable nuclides.

Alpha particles are composed of two protons and two neutrons. Thus they have Z = 2, N = 2, and A = 4 and correspond to a helium nucleus He. The emission of a particles thus produces a decrease of 4 units in A. An unstable nuclide undergoing a decay may emit a particles of various energy and thus directly reach the ground level of the stable product. Alternatively, as in )3 emission, an intermediate excited state is reached, followed by y emission. Figure 11.7 shows, for example, the decay process of ioTh., which may directly attain the ground level of by emission of a particles of energy 5.421 MeV or intermediate excited states by emission of a particles of lower energy, followed by y emission. 

There are many examples of first-order reactions. The most often encountered in geochemistry is the radioactive decay of an unstable nuclide. For example, the rate law for the decay of " Sm (Reaction 1-2) can be written as... 

The decay (ot- and p-decay) of an unstable nuclide can be described by the decay equation (first-order reaction) ... 

Long-lived unstable nuclide naturally occurring... 

Short-lived unstable nuclide not occurring naturally... 

Schematic presentation of the Chart of the Nuclides, which plots Z (number of protons) versus N (number of neutrons). Stable isotopes, shown in black, define a narrow band within a wider band of unstable nuclides. In general, elements with even Z are more abundant and have more isotopes than elements with odd Z. Among the isotopes of a given element, those with even N are more abundant than those with odd N.

Detailed view of a portion of the Chart of the Nuclides. Stable isotopes are shaded. For stable nuclides, isotopic abundances are given below the element symbol and isotopic masses are given at the bottom of the square. Half-lives of the unstable nuclides are given, along with their decay modes. 

Portion of the Chart of the Nuclides showing s-process and r-process pathways. The s-process pathway, shown by the dark line in the center of the valley of p-stability, shows how a nuclide that successively captures individual neutrons would evolve. Each added neutron moves the nuclide to the right on the diagram, until it reaches an unstable nuclide, in which case it will p-decay to the stable nuclide with a higher Z. In contrast, in situations where nuclides capture neutrons very rapidly ( -process), they will be driven far to the right of the valley of p-stability until the timescale for neutron capture matches that for p-decay. They will then move to higher Z and capture more neutrons until they either reach a size that causes them to fission (break) into smaller nuclei (which can then capture more neutrons) or until the neutrons disappear, in which case they will p-decay back to the first stable isotope along paths of constant A (arrows). 

Radioactive decay usually involves one of three basic types of decay, a decay, (3 decay, or y decay in which an unstable nuclide spontaneously changes into a more stable form and emits some radiation. In Table 1.1, we summarize the basic features of these decay types. 

Consider the two nuclides of mass number 7, 7Li and 7Be. Which of the two is the more stable How does the unstable nuclide decay into the more stable one ... 

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