Stable nuclide

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. 

Figure 11,1 Chart of nuclides. stable, unstable. Reproduced with modihcations from Rankama (1954). 

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. 

The negative quantity in brackets is an irrational number known as the golden ratio, t = 0.61803. The solution Z = — iV(1.61803...) = — nuclide stability, as defined on both plots, converges to a point on the line r = Nx = r at A 267, the maxinum possible mass number for nuclides, stable against /1-type decay. By definition, this maximum,... 

The amount of a nuclide (stable or radioactive) relative to other nuclides of the same element in a given sample. The natural abundance is the abundance of a nuclide as it occurs naturally. For instance, chlorine has two stable isotopes of masses... 

The amount of a nuclide (stable or radioactive) relative to other nuclides of the same element in a given sample. 

Natural abundance. The natural abundances listed are on an atom percent basis for the stable nuclides present in naturally occurring elements in the earth s crust. 

Parent Half-fife, yr Stable daughter Natural abundance of daughter, % Intermediate nuclides... 

It will be recalled that is 100% abundant and is the heaviest stable nuclide of any element (p. 550), but it is essential to use very high purity Bi to prevent unwanted nuclear side-reactions which would contaminate the product Po in particular Sc, Ag, As, Sb and Te must be <0.1 ppm and Fe <10ppm. Polonium can be obtained directly in milligram amounts by fractional vacuum distillation from the metallic bismuth. Alternatively, it can be deposited spontaneously by electrochemical replacement onto the surface of a less electropositive metal... 

Element has no stable nuclides the value given in parentheses is the atomic mass number of the isotope of longest known half-life. However, three such elements (Th, Pa and U) do have a characteristic terrestrial isotopic composition, and for these an atomic weight is tabulated. 

We can use Fig. 17.13 to predict the type of disintegration that a radioactive nuclide is likely to undergo. Nuclei that lie above the band of stability are neutron rich they have a high proportion of neutrons. These nuclei tend to decay in such a way that the final n/p ratio is closer to that found in the band of stability. For example, a l4C nucleus can reach a more stable state by ejecting a (3 particle, which reduces the n/p ratio as a result of the conversion of a neutron into a proton (Fig. 17.15) ... 

FIGURE 17.12 The numbers of stable nuclides having even or odd numbers of neutrons and protons. With the exception of hydrogen, by far the greatest number of stable nuclides (157) have even numbers of both protons and neutrons. Only four stable nuclides have odd numbers of both protons and neutrons. 

Very few nuclides with Z < 60 emit a particles. All nuclei with Z > 82 are unstable and decay mainly by a-particle emission. They must discard protons to reduce their atomic number and generally need to lose neutrons, too. These nuclei decay in a step-by-step manner and give rise to a radioactive series, a characteristic sequence of nuclides (Fig. 17.16). First, one a particle is ejected, then another a particle or a (3-particle is ejected, and so on, until a stable nucleus, such as an iso tope of lead (with the magic atomic number 82) is formed. For example, the uranium-238 series ends at lead-206, the uranium-235 series ends at lead-207, and the thorium-232 series ends at lead-208. 

This equation shows that, the larger the value of k, the shorter the half-life of the nuclide. Nuclides with short half-lives are less stable than nuclides with long half-... 

A radioactive isotope X with a half-life of 27.4 d decays into another radioactive isotope Y with a half-life of 18.7 d, which decays into the stable isotope Z. Set up and solve the rate laws for the amounts of the three nuclides as a function of time, and plot your results as a graph. 

Eigure 22-2 illustrates this process schematically for fluorine 9p + 10iH F Because any stable nucleus is more stable than its separated nucleons, nuclear formation reactions of all stable nuclides are exothermic. 

The energy change is negative, indicating that this helium nuclide is more stable than its separate component particles. We expect this for any stable nuclide. 

Plot of the binding energy per nucleon vs. mass number A. The most stable nuclides lie in the region around... 

Although nuclides with mass numbers around 60 are the most stable, the balance of electrical repulsion and strong nuclear attraction makes many combinations of protons and neutrons stable for indefinite times. Nevertheless, many other combinations decompose spontaneously. For example, all hydrogen nuclides with j4 > 2 are so... 

As described in Chapter 2 (see Figure ), stable nuclides fall within a belt of stability with roughly equal numbers of neutrons and protons. Lighter nuclides lie along the = Z line, but as the mass of the nuclide increases, the... 

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