What part of speech is the word "not"?

It's a no brainer that "not" is always referred to as an adverb. Look in any dictionary and that becomes clear. However, it cannot be denied that when identifying a part of speech we must look at how it functions in the sentence in which it is being used. In the following sentence, it simply does not work as an adverb and works only as an adjective.

She should have used those gift cards before January 1, 2008 because not one of them now has a balance.

One might argue that the sentence could be changed to read: "...because now one of them has not a balance," but to do so inherently changes the intended meaning of the sentence. As rewritten, only one of the gift cards does not have a balance. Therefore, in this sentence, "not" is modifying the pronoun "one" by telling us "how many" gift cards have a balance and, in this case, zero of them do. "How many, which one, and what kind" all point to a word being a descriptive word, also known as an adjective.

There is a nice debate going on on http://www.usingenglish.com/forum/advanced/1502-not-i.html
as to whether "not" is an adverb or a particle.

Adverbs are major parts of speech, as are nouns, verbs, adjectives and prepositions.
so: It was not good- "not" is being used as an adverb

Particles, on the other hand, are defined as minor parts of speech, especially short undeclinable ones, like the ones we see attached to phrasal verbs.
so: Who are the cake? Not I/me. "Not" is being used as a particle.

Interesting to follow the debate. I would have just said it was a negater, as that is what I use when teaching LOTE.(Languages Other Than English)

Adverb. Modifies a verb, adjective, or other adverb.

it is an adverb

Not is an adverb in my opinion.However, please ponder this thought:
My question is for people who consider "cannot" the acceptable way to write "can not." I have always written "can not" as two words. I consider it a helping verb + an adverb that comes before an action verb that follows the word "not." What part of speech then is "cannot" when it is written as one word? Can you make it past tense? (NO! -So how is it correct to write it as one word????)

I consider it an entirely different part of speech, as a polarity or negator, separate from adverbs, adjectives, nouns, etc. since it has the ability to act like so many things.

Consider the phrase: "I always eat." "Always" here is used as an adverb. Now watch watch what happens.

"I not eat." In this case, it CAN'T be an adverb. It CAN act as an adverb, and it CAN act like a particle, adjective, conjunction, and sometimes a noun too, but it's NOT (no pun intended).

I'm currently doing an independent study of this in my college linguistics class and have gotten some support from my professors about this topic. I may contribute to this topic again in the future, as I gather more data.

Though we think "not one" is cute or emphasizes things, it is grammatically incorrect. It should be none and that solves the dilemma about "not".

Referring back to Mrs. B's comment, it does matter how a word is used, to correctly parse it, unfortunately, even in the example she gave, it was still an adverb (because it was modifying an adjective). "For an example of how 'not' can be a noun, just re-read this sentence."