An isotope referred to as an element rather than as part of the name of a chemical compound may be described at first mention by providing the name of the element spelled out followed by the isotope number in the same typeface and type size (no hyphen, subscript, or superscript is used). The element abbreviation may be listed in parentheses at first mention and used thereafter in the article, with the isotope number preceding the element symbol as a superscript.
Of the 13 known isotopes of iodine, only iodine 128 (128I) is not radioactive. The investigators used 128I to avoid the difficulty and expense of disposing of radioactive waste.
The symbol representing a single element should not be used as an abbreviation for a compound (eg, do not abbreviate the compound sodium arsenate As 74 as 74As).