Hm... I don't personally think that Atheists in general believe that there isn't a god (as a definite claim), but that there isn't a compelling reason to think that there is one. I realize that it's not often phrased that way because we (well, I and therefore everyone else...) tend to be sloppy with our language.
Is that self-imposed? You betcha!
Theists are willing to accept a god thingy evidence be damned, but in general that only goes for the supernatural. Whenever I tell them that I have a patent that's about to bring in millions, and I'll split it with them if they sell everything they own and give me the proceeds now, they suddenly need proof to believe...
As for neutrons, and other aspects of physics, except for the ones with books to sell, most scientists accept certain things as "proven" because they are so successful at modeling the universe. However, if a situation came along that, say, a particle theory with electrons gives the wrong answer to, but another theory (without e-) gives the correct answer to as well as correctly handling what the theory with e- handles, then the e- theory would be abandoned in favor of the more successful theory, in spite of the fact that e- are already "proven." That may be a run-on sentence...
Anyway, we don't doubt those things because there are compeling reasons, usually extraordinarily compelling reasons, to believe them, not because we know for absolute fact that they exist.
Personally, as much empasis as religions in general seem to place of faith, if they're right, I'd be shocked if these god thingies arranged the universe so that they can be shown to exist for certian. So, it seems to me that there are just fundamentally different people in the world: those who are willing to blindly belive and those who aren't.
As for athiests being arrogant because we reject the various gods currently on the table, we just believe in one less god than you, so you're only a half-step down from us arrogance wise... :wink: