Ok, wierd question about fairly odd parents and atoms?

ok...lol..this is going to sound dum..but on the fairly fairly oddparents movie where its timmy's bday and he gets a muffin or something and he splits and atom and it blows up..why is this? i thought when an atom is split it just becomes two seperate atoms...? idk..my little sister wants to know and she wont shut up about it