Atheism is the "lack of a belief in God". It's not the belief that there is no God, it's the lack of a belief that there is one. All agnostics are also atheists.
https://en.wikipedia.org/wiki/Atheism
"Atheism is, in the broadest sense, the absence of belief in the existence of deities. Less broadly, atheism is the rejection of belief that any deities exist. In an even narrower sense, atheism is specifically the position that there are no deities. Atheism is contrasted with theism, which, in its most general form, is the belief that at least one deity exists."
Of course not. Absence of belief is not belief in absence. If "not believing in something" were an active task, I'd be busy all day not-believing in trillions of bizarre things.