Also to help understand this concept there is a chart of the nuclides, known as a Segre chart. To determine the stability of an isotope you can use the ratio neutron/proton (N/Z). To identify the stability of an isotope it is needed to find the ratio of neutrons to protons. Nuclear stability is a concept that helps to identify the stability of an isotope. Neutron and Atomic Numbers and Nuclear Stability On the other hand, nuclei with an odd number of protons and neutrons are mostly unstable. Heavy nuclei with an even number of protons and an even number of neutrons are (due to Pauli exclusion principle) very stable thanks to the occurrence of ‘paired spin’. For example, actinides with odd neutron number are usually fissile (fissionable with slow neutrons) while actinides with even neutron number are usually not fissile (but are fissionable with fast neutrons). It must be noted, especially nuclear cross-sections may vary by many orders from nuclide with the neutron number N to nuclide with the neutron number N+1. Properties of atomic nuclei (atomic mass, nuclear cross-sections) are determined by the number of protons and number of neutrons (neutron number). Neutron and Mass Numbers and Nuclear Properties For example, the neutron number of uranium-238 is 238-92=146. We can determine the neutron number of certain isotope. Therefore, we cannot determine the neutron number of uranium, for example. Each nuclide is denoted by chemical symbol of the element (this specifies Z) with tha atomic mass number as supescript. The various species of atoms whose nuclei contain particular numbers of protons and neutrons are called nuclides. Nuclides that have the same neutron number but a different proton number are called isotones. Neutron number is rarely written explicitly in nuclide symbol notation, but appears as a subscript to the right of the element symbol. The difference between the neutron number and the atomic number is known as the neutron excess: D = N – Z = A – 2Z. Neutron number plus atomic number equals atomic mass number: N+Z=A. The total number of neutrons in the nucleus of an atom is called the neutron number of the atom and is given the symbol N. Mass numbers of typical isotopes of Silicon are 28 29 30. Neutron Number and Mass Number of Silicon
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