Soda (n.) late 15c., "sodium carbonate," an alkaline substance extracted from certain ashes (now made artificially), from Italian sida (or Medieval Latin soda) "a kind of saltwort," from which soda was obtained, of uncertain origin. Perhaps it is from a Catalan sosa, attested from late 13c., of uncertain origin. Proposed Arabic sources in a name of a variety of saltwort have not been attested and that theory is no longer considered valid. Another theory, considered far-fetched in some quarters, traces it to Medieval Latin sodanum "a headache remedy," ultimately from Arabic suda "splitting headache."

Source : Online Etymology Dictionary

The element Sodium is named via its extraction from soda as above.

Note: The Online Etymology Dictionary maintains a free-access list of the origins of thousands of English words, a significant proportion of which are unknown