The only free and comprehensive online etymological dictionary of the Spanish language
radio- Prefix "radiation"

From Latin radius "ray" (see rayo).
radio (1) m. (Noun) "radio"

From radiotelegrafía.
radio (2) m. (Noun) "radius"

18th cent. From Latin radius "ray," but also "wheel spoke" which bridges the distance and position of a geometric radius. See rayo for continued etymology.
radio (3) m. (Noun) "radium"

A modern Latinism from radium "radiation," "ray." See rayo for continued etymology.
radiotelegrafía f. (Noun) "radiography"

From radio- "radiation" and grafia "writing."
rápido (Adjective) "quick"

17th cent. borrowing from Latin rapidus "quick," but literally "seized," an adjective from rapere "to snatch" (see raptar).

Romance: Western Vulgar Latin: Asturian rápidu, Portuguese rápido, French rapide, Italian rapido ; Eastern Vulgar Latin: Romanian rapid

Corominas (1991) notes that rápido cannot have been borrowed earlier than the 17th cent., as the Spanish-Italian dictionary Vocabulario de las dos lenguas Italiano y Castellano by Cristóbal de las Casas (c. 1570) translates the Italian rapido as "arrebatado, ligero;" the word rapido evidently absent from his Spanish wordbank.
raptar (Verb) "to kidnap"

Early 20th cent. borrowing from Latin raptare "to kidnap," a verb formed from raptus "seized," the passive participle of rapere "to snatch," "to kidnap." From Proto-Italic *rap-i- 'id.' From Proto-Indo-European *h1rp-i̯- 'id.'

Romance: Western Vulgar Latin: Portuguese raptar

Indo-European: Balto-Slavic: Lithuanian ap-rė́pti "to take;" Albanian rjep "to tear off;" Hellenic: Ancient Greek ἐρέπτομαι (eréptomai) "to devour"
raro, ralo (Adjective) "strange;" "rare"

14th cent. Old Spanish ralo; very late 15th cent. raro. The intervocalic dental was uncertain, curiously leading to two native reflexes of the same Latin word. Raro and ralo were free variants until Modern Spanish and never distinctly separate. From Latin rarus "scattered," "rare," "thin." Of unknown origin.

Romance: Western Vulgar Latin: Asturian ralu, raru, Portuguese ralo, raro, Galician raro, Catalan rar, French rare, Italian raro ; Eastern Vulgar Latin: Aromanian rar, Romanian rar ; Sardinian: raru
rata (1) f. (Noun) "rat"

13th cent. From Vulgar Latin *ratta 'id.' Borrowed from Proto-West Germanic or Proto-Germanic *ratta- 'id.' According to Kroonen (2011), a pre-form *Hrót-ēn- is reconstructible, but ultimately of unknown origin.

Germanic: West Germanic: Old High German ratto "rat," Old Saxon ratta 'id.,' English rat
rata (2) f. (Noun) "proportional part"

13th cent. From Latin rata, feminine of ratus "established." From Proto-Italic *rato- "thought." From Proto-Indo-European *Hr̥h1-to- "counted."

Indo-European: Germanic: Gothic raþjo "number," Old High German redea "account," Old Saxon rethia 'id.'