How can you atheist explain there is no god when..............?

The earth formed there was obviously something that helped form it. I know you will say the big bang theory and stuff like that but what produced the 1st atom in the world to allow all of this to happen? I dont think god formed anything but I think he exist because there had to be someone to creat the 1st neccasary element in this world to form everything else. Explain how you could say he doesnt exist now.