Articles are classified as "articles" in the 9 parts of speech system. Those who recognize only 8 parts of speech classify articles as adjectives.
The only word that doesn't fit the classification scheme is the "to" in a "to infinitive. Most people call that a "particle". Because that is not part of the parts of speech system, many dictionaries list it under "preposition", but it is clearly not a prepositiion, IMO.
Some people also use "particle" for the preposition or adverb part of phrasal verbs. Perhaps "particle" will become the 10th part of speech.