Q. How should the symbols N₂ and O₂ be pluralized in Chicago style? N₂’s and O₂’s or italicized symbols with no apostrophes?
A. To channel Bartleby (Melville’s fictional nineteenth-century scrivener), we prefer not to write chemical formulas as plurals; nor would we apply italics (as we would for ordinary letters as letters; see CMOS 7.67). We’d advise rewording instead (e.g., “two N₂ molecules”). But if you absolutely must express a molecule as a plural, an apostrophe will help make it clear that the s isn’t part of the formula. It may not be precisely Chicago style, but readers will know what you mean—which is the goal of all good editing.