Does the “2” in SN2 denote that there is two species involved in the reaction? Is one of these the nucleophile? (Yes to both. - GS). Just to be clear, for an SN2 reaction there would be (for example) a methyl iodide molecule and a nucleophile? (Yes. - GS).  Or would there be a methyl iodide molecule, an ethoxide ion, AND a nucleophile? (No. Methoxide is a nucleophile. - GS).