The larger elements are stratified at the core of the Jovian planet. The gravitational force is so large that part of the core consists of liquid hydrogen. That which cannot be captured by the core is in the upper regions in gaseous form.
Helium is one of the noble gases (the lightest one) and has an extremely low melting point. It addition it will combine with absolutely nothing that I know of. So helium has found itself in the upper regions of the planet as well.
