No one really knows what happens inside an atom. But two competing groups of scientists think they’ve figured it out. And both are racing to prove that their own vision is correct.

Here’s what we know for sure: Electrons whiz around “orbitals” in an atom’s outer shell. Then there’s a whole lot of empty space. And then, right in the center of that space, there’s a tiny nucleus — a dense knot of protons and neutrons that give the atom most of its mass. Those protons and neutrons cluster together, bound by what’s called the strong force. And the numbers of those protons and neutrons determine whether the atom is iron or oxygen or xenon, and whether it’s radioactive or stable.