Agnosticism is the belief that it is impossible to know if there is a god

Atheism is the lack of belief that there is a god

Some atheists might assert that they know there is no god, but that is not the meaning of atheism, though some people call that "strong atheism"

On the flip side, one can be agnostic and think there is a god.

Being both atheist and agnostic (what some people call weak atheism) is the rational position given that there is no evidence for any god (or other supernatural phenomenon) existing, but that you also can't design an experiment that the result of would let you conclude a god (or other supernatural phenomenon) does not exist.

Occam's razor doesn't have anything to do with it. It is simply irrational to leap to believing things for which there is no evidence.