Neutron-Poor Nuclei Below the Band of Stability

The sum of the atomic numbers (the left subscript in the nuclide symbol) of the reactants must equal the sum of the atomic numbers of the products this maintains charge balance. 

Because such equations are intended to describe only the changes in the nucleus, they do not ordinarily include ionic charges (which are due to changes in the arrangements of electrons). In the following sections, we will see many examples of such equations for nuclear reactions. For several of these, the check for mass balance and charge balance is illustrated in the margin. 

22-6 Neutron-Rich Nuclei (Above the Band of Stability) 

Nuclei in this region have too high a ratio of neutrons to protons. They undergo decays that decrease the ratio. The most common such decay is beta emission. A beta particle is an electron ejectedthe nucleus when a neutron is converted into a proton. 

beta emission results in an increase of one in the number of protons (the atomic number) and a decrease of one in the number of neutrons, with no change in mass number. Examples of beta-particle emission are 

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NEUTRON-POOR NUCLEI (BELOW THE BAND OF STABILITY)... 

Positron emission and eiectron capture For nuclei with low neutron-to-proton ratios lying below the band of stability, there are two common radioactive decay processes that occur, positron emission and electron capture. These two processes tend to increase the neutron-to-proton ratio of the neutron-poor atom. After an unstable atom undergoes electron capture or positron emission, the resulting atom is closer to, if not within, the band of stability. 

Figure 22-1 A plot of the number of neutrons, A/, versus the number of protons,Z, in nuclei.The stable nuclei (green dots) are located in an area known as the band of stability. All other nuclei in the white, pink, and blue regions are unstable and radioactive. No nuclei exist in the large gray shaded region. Most unstable, radioactive nuclei occur outside the band of stability. As atomic number increases, the N/Z ratio of the stable nuclei increases. Unstable nuclei above the band of stability are referred to as neutron-rich nuclei (Woe shad/ng) those below the band of stability are called neutron-poor nuclei (pinkshading). Unstable (radioactive) nuclei decay by alpha emission, beta emission, positron emission, or electron capture. Lighter neutron-poor nuclei usually decay by positron emission or electron capture, either of which converts a proton into a neutron. Heavier neutron-poor nuclei usually decay by alpha emission, which decreases the neutron/proton ratio. Neutron-rich nuclei decay by beta emission, which transforms a neutron into a proton. Decay by alpha emission is by far the most predominant mode of decay for nuclei with atomic numbers beyond 83 (bismuth).

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