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The stability of any particular isotope depends on a variety of factors, and no single rule allows us to predict whether a particular nucleus is radioactive and how it might decay. There are, however, several generalizations that can be made about the manner in which radioisotopes will decay.
As you know, like charges repel each other, and thus it may seem surprising that a large number of protons (all positively charged) can reside within the very tiny nucleus. There are two another forces called the strong and weak nuclear forces that exists between the protons and neutrons. Thus the nucleus is held together by a delicate balance between the + + repulsion of protons and the close distance attraction of the nuclear strong and weak forces. All atoms with more than one proton have neutrons. Stable atoms with up to 20 protons tend to have the same number of neutrons as protons. For nuclei with higher than 20 atomic numbers, the number of neutrons exceeds the number of protons. Indeed, the number of neutrons necessary to create a stable nucleus increases more rapidly than the number of protons. Thus the neutron / proton ratio of stable atoms increases with increasing atomic number.
The band of orange dots in the band on the graph to the right represent the belt of stability – the isotopes which have a stable amount of protons and neutrons. The dashed line represents a one neutron to one proton ratio. Clearly as atoms get bigger, more neutrons are necessary to hold the nucleus together. The belt of stability ends at element 83. All nuclei with 84 of more protons are radioactive.
There are three situations that represent unstable nuclei, represented by the blue squares on the graph.
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1 Nuclei above the belt of stability – too many neutrons for not enough protons. These neutronrich nuclei can lower their n / p ratio and move toward the belt of stability by emitting a beta particle. Beta emission decreases the number of neutrons and increases the number of protons
(increases the atomic #).
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2 Nuclei below the belt of stability – not enough neutrons for too many protons. These protonrich nuclei can increase their n / p ratio by either positron emission or electron capture. positron emission of electron capture will increase the number of neutrons and decrease the number of protons (decreases the atomic #).
3 Nuclei beyond the belt of stability – too many protons and too many neutrons. Any isotope with an atomic number ≥ 84 needs to lose both protons and neutrons and thus tends to undergo alpha emission, which will reduce both the protons and neutrons by 2 each (Decreases both the atomic # and the mass number).
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In the sketch to the right that represents a section of the graph above, the blue squares represent unstable isotopes. The pink circle represents any stable isotope on the belt of stability. The arrows represent the type of nuclear decay that will solve the instability of the nucleus.
Beta decay:
Loss of 0
-1 e
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Alpha decay:
Loss of 4
2
He
3
2
Positron emission or electron capture:
Loss of 0
+1 e or gain of 0
-1 e