I've got a couple of related jokes that aren't necessarily funny but they are very clean. You can tell these to anyone of any age, as long as they are old enough to understand components of atoms.
Joke #1: An atom walks into a bar and slides up onto a barstool, looking a bit down. The bartender asks, "Why are you looking so sad?" The atom replies, "I think I've lost an electron." The bartender says, "Are you sure?" and the atom replies, "Yeah, I'm positive."
Joke #2: A neutron walks into a bar and asks the bartender, "How much for a pint of beer?" The bartender replies, "For you? No charge!"
Ba-dum-bum!