![]() ![]() 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’. The nuclear properties (atomic mass, nuclear cross-sections) of the element are determined by the number of protons (atomic number) and number of neutrons (neutron number). 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). In the periodic table, the elements are listed in order of increasing atomic number Z. The number of electrons in each element’s electron shells, particularly the outermost valence shell, is the primary factor in determining its chemical bonding behavior. The configuration of these electrons follows from the principles of quantum mechanics. The chemical properties of the atom are determined by the number of protons, in fact, by number and arrangement of electrons. 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 protons and neutrons in the nucleus of an atom is called the atomic mass number (or the mass number) of the atom and is given the symbol A. The nucleus is composed of protons and neutrons. The atom consist of a small but massive nucleus surrounded by a cloud of rapidly moving electrons.
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