The rates, and some derived parameters, of reaction of three heavy atom neutral nucleophiles (thiourea, N-acetylthiourea, and selenourea) with 1-fluoro- and 1-chloro-2,4-dinitrobenzene are measured and used to estimate (a) the mobilities of fluorine relative to chlorine, an important mechanistic feature of the usual (addition-elimination) SNAr reactions; and (b) the strength of the reagents. Results are compared with those for a range of neutral and anionic light (Periodic Table first row) and heavy atom (Periodic Table second or higher row) nucleophiles. Results for 1-iodo-2,4-dinitrobenzene are included where available. A marked reduction in F:Cl mobility ratios (which are matched by F:heavy halogen ratios), well known in following a sequence from light to heavy atom anionic nucleophiles, is now shown to occur also with neutral nucleophiles. 1115531557