The main building blocks of atomic nuclei are protons and neutrons, and it is convenient to describe the composition of an atom in terms of the number of protons and neutrons present in its nucleus. The number of protons contained in the nucleus of an atom is called the atomic number (Z) and the number of neutrons is the neutron number (N). Because the masses of these particles are each approximately equal to one mass unit, the total number of protons plus neutrons is called the mass number (A). The electron mass is about 1825 times smaller than the proton mass and, therefore, can be neglected in calculating the mass number. For any element, the mass number is equal to the atomic weight rounded off to the nearest integer value.
The proton has a positive charge that is equal in magnitude, but opposite in polarity, to the charge of an electron. The number of protons defines how many extranuclear electrons an atom can have under conditions of electric neutrality. It is the number of...
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© 1998 Kluwer Academic Publishers
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Harmon, R.S. (1998). Atomic number . In: Geochemistry. Encyclopedia of Earth Science. Springer, Dordrecht. https://doi.org/10.1007/1-4020-4496-8_19
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DOI: https://doi.org/10.1007/1-4020-4496-8_19
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