How are you certain that there is no God? Have you searched throughout the entire universe? Do you possess omniscience? If not, then you cannot be certain of such a universal negative, and therefor your assertion of certainty is groundless and false.
You cannot account for morality apart from God. You cannot derive an "ought" merely from what is, which in your view, is nothing but matter in motion. There are no "good" and "bad" atoms.
Cordially,
And don't worry - you evidence sufficient cluelessness of what I believe that there is no chance I can explain it to you in a way that you find plausible, much less persuasive.
I have searched far and wide for authors that might reflect my beliefs, and describe them better than can I. The closest I've found is Daniel Dennett.