Note that, each element may contain more isotopes, therefore this resulting atomic mass is calculated from naturally-occuring isotopes and their abundance. The atomic mass is carried by the atomic nucleus, which occupies only about 10 -12 of the total volume of the atom or less, but it contains all the positive charge and at least 99.95% of the total mass of the atom. The atomic mass or relative isotopic mass refers to the mass of a single particle, and therefore is tied to a certain specific isotope of an element. Many other rare types of decay, such as spontaneous fission or neutron emission are known. Unstable isotopes decay through various radioactive decay pathways, most commonly alpha decay, beta decay, gamma decay or electron capture. If there are too many or too few neutrons for a given number of protons, the resulting nucleus is not stable and it undergoes radioactive decay. As a result, as the number of protons increases, an increasing ratio of neutrons to protons is needed to form a stable nucleus. Neutrons stabilize the nucleus, because they attract each other and protons, which helps offset the electrical repulsion between protons. There are only certain combinations of neutrons and protons, which forms stable nuclei. These two forces compete, leading to various stability of nuclei. The excess neutrons act somewhat like nuclear glue. Only two stable nuclides have fewer neutrons than protons: hydrogen-1 and helium-3.Ītomic nuclei consist of protons and neutrons, which attract each other through the nuclear force, while protons repel each other via the electric force due to their positive charge. These extra neutrons are necessary for stability of the heavier nuclei. It can be observed from the chart that there are more neutrons than protons in nuclides with Z greater than about 20 (Calcium). This chart shows a plot of the known nuclides as a function of their atomic and neutron numbers. 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 Sodium are 23.
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