dense, rare metallic element, 1925, Modern Latin, from Latin Rhenus "the river Rhine" (see Rhine) + element ending -ium. Coined by German chemists Walter Noddack and his wife Ida Tacke; they named it in honor of her native region.
principal river in western Germany, from German Rhein, from Middle High German Rin, ultimately from Gaulish Renos, literally "that which flows" (from PIE root *rei- "to run, flow"). The spelling with -h- (also in Latin Rhenus; French Rhin) is from influence of the Greek form of the name, Rhenos.
word-forming element in chemistry, used to coin element names, from Latin adjectival suffix -ium (neuter of -ius), which formed metal names in Latin (ferrum "iron," aurum "gold," etc.). In late 18c chemists began to pay attention to the naming of their substances with words that indicate their chemical properties. Berzelius in 1811 proposed forming all element names in Modern Latin. As the names of some recently discovered metallic elements already were in Latin form (uranium, chromium, borium, etc.), the pattern of naming metallic elements in -ium or -um was maintained (in cadmium, lithium, plutonium, etc.; helium is an anomaly).
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Harper, D. (n.d.). Etymology of rhenium. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/rhenium