metallic element, 1800, coined 1798 in Modern Latin by German chemist and mineralogist Martin Heinrich Klaproth (1743-1817) from Latin tellus (genitive telluris) "earth" (see tellurian). With metallic element ending -ium.
"pertaining to the earth," 1846, from -ian + Latin tellus (genitive telluris) "earth, land, ground; the earth" (related to Tellus, Roman goddess of the earth), probably from PIE root *telho- "ground, floor" (source also of Sanskrit talam "plain, sole of the foot;" Greek telia "dice board;" Latvian telint "to spread out;" Lithuanian tils "bottom of a barge, flooring," patalas "bed;" Old Prussian tallus "floor;" Old Church Slavonic tilo "floor;" Russian potolok "ceiling;" Old Irish talam "earth;" Old Norse ilja, Middle Dutch dele "plank"). Or possibly from PIE *telh- "to bear." As a noun, "inhabitant of Earth" (with reference to supposed inhabitants of other worlds) from 1847.
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).