Historical origin:
The forms and that combines with feminine words that begin by a or (ha) tonic are historically feminine, since it wass Latin origin also feminine illam (> ell> el(a)), unam (> un(a)). Therefore, it is necessary to speak of forms, masculine ones el, un (el niño, un niño), and other ones feminine (el alma, un alma).

General Rule:
To the feminine nouns that begin by the phoneme /a/ or /ha/ tonic takes the article el or un instead of la or una for following conditions:
a) the article goes immediately in front of the noun.
b) this noun belongs to the femenine and is in singular c) this noun begins by a tonic phoneme /a/ or /ha/.
el agua clara, el arma blanca, el hacha pesada

a) a) The letters of the alphabet: la hache, la a .
b) When the determine or indetermine article doesn't precede immediately to the feminine noun that begins by the tonic phoneme /a/, is used the feminine form/la / una:
la pequeña hacha, una altiva águila, la única arma
c) When the feminine noun begins by the atonic phoneme /a/ or /ha/:
la alegría, una avispa, la almeja
d) When the noun is a woman name: la Angela, la Ana.
e) For the city La Haya.
f) When the article designates the genus of a person or an animal:
el árabe (man), la árabe (woman), el ánade (male), la anade (female)
g) When the word that begins by the tonic phoneme /a/ is not a noun, but an adjective:
la agria polémica > una agria polémica
All the feminine nouns that begin with a / ha tonics and which they go preceded in singular by articles el or un, they continue being feminine in singular, thus like in plural:
el agua clara, las aguas claras, un hacha pesada, una hachas pesadas, el ala blanca, las alas blancas