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Atoms having the same number of protons (and therefore the same number of
electrons) but different numbers of neutrons
are called isotopes of the element in question. Thus, the isotopes
of an element have
the same atomic number but differ in their atomic mass number. A compact
notation for isotopes of an element is illustrated by the following examples.
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In this notation the element is represented by its chemical symbol, the atomic number is denoted by a lower left subscript, the number of neutrons is denoted by a lower right subscript, and the atomic mass number is denoted by an upper left superscript (some of these superscripts and subscripts may be omitted, depending on the context).