Etymology
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valence (n.)
early 15c., "extract, preparation," from Latin valentia "strength, capacity," from valentem (nominative valens) "strong, stout, vigorous, powerful," present participle of valere "be strong" (from PIE root *wal- "to be strong"). Chemistry sense of "relative combining capacity of an element with other atoms when forming compounds or molecules" is recorded from 1884, from German Valenz (1868), from the Latin word. Related: Valency.
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multivalent (adj.)

1869, originally in chemistry, "having more than one degree of valency," from multi- "many" + -valent (see valence in the chemistry sense). Related Multivalence.

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bivalent (adj.)
1864, of chemicals, 1899, of chromosomes, from bi- + -valent (see valence in the chemistry sense).
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covalent (adj.)

1927, from covalence "the linking of two atoms by a shared pair of electrons" (1919), from co- + valence.

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Valencia 
place in Spain, Roman Valentia Edetanorum "fort of the Edetani," a local people name; the first element from Latin valentia "strength" (see valence (n.)).
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polyvalent (adj.)

"capable of combining partly with one element, partly with another," 1881, from poly- + -valent (see valence in the chemistry sense). Coined by German chemist Emil Erlenmeyer, who also designed the flask that bears his name.

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*wal- 

Proto-Indo-European root meaning "to be strong."

It forms all or part of: ambivalence; Arnold; avail; bivalent; convalesce; countervail; Donald; equivalent; evaluation; Gerald; Harold; invalid (adj.1) "not strong, infirm;" invalid (adj.2) "of no legal force;" Isold; multivalent; polyvalent; prevalent; prevail; Reynold; Ronald; valediction; valence; Valerie; valetudinarian; valiance; valiant; valid; valor; value; Vladimir; Walter; wield.

It is the hypothetical source of/evidence for its existence is provided by: Latin valere "be strong, be well, be worth;" Old Church Slavonic vlasti "to rule over;" Lithuanian valdyti "to have power;" Celtic *walos- "ruler," Old Irish flaith "dominion," Welsh gallu "to be able;" Old English wealdan "to rule," Old High German -walt, -wald "power" (in personal names), Old Norse valdr "ruler."

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Valentine (n.)

mid-15c., "sweetheart chosen on St. Valentine's Day," from Late Latin Valentinus, the name of two early Italian saints (from Latin valentia "strength, capacity;" see valence). Choosing a sweetheart on this day originated 14c. as a custom in English and French court circles. Meaning "letter or card sent to a sweetheart" first recorded 1824. The romantic association of the day is said to be from it being around the time when birds choose their mates.

For this was on seynt Volantynys day
Whan euery bryd cometh there to chese his make.
[Chaucer, "Parlement of Foules," c. 1381]

Probably the date was the informal first day of spring in whatever French region invented the custom (many surviving medieval calendars reckon the start of spring on the 7th or 22nd of February). No evidence connects it with the Roman Lupercalia (an 18c. theory) or to any romantic or avian quality in either of the saints. The custom of sending special cards or letters on this date flourished in England c. 1840-1870, declined around the turn of the 20th century, and revived 1920s.

To speak of the particular Customs of the English Britons, I shall begin with Valentine's Day, Feb. 14. when young Men and Maidens get their several Names writ down upon Scrolls of Paper rolled up, and lay 'em asunder, the Men drawing the Maidens Names, and these the Mens; upon which, the Men salute their chosen Valentines and present them with Gloves, &c. This Custom (which sometimes introduces a Match) is grounded upon the Instinct of Animals, which about this Time of the Year, feeling a new Heat by the approach of the Sun, begin to couple. ["The Present State of Great Britain and Ireland" London, 1723]
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valance (n.)
piece of hanging, decorative drapery, mid-15c., of uncertain origin, probably from Anglo-French *valaunce, valence, from valer "go down, let down," variant of Old French avaler "descend, go down;" or possibly from the plural of Old French avalant, from present participle of avaler "go down." The notion is of something "hanging down." Not now considered to be from the name of Valence in southwestern France, which is from the Roman personal name Valentius. Related: Valenced.
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ferric (adj.)
1799, "pertaining to or extracted from iron," from Latin ferrum "iron" (see ferro-) + -ic. Especially of iron with a valence of three.
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