The depiction above implies how weakly rubidum holds its outermost electron and why it so readily gives it up to become Rb+. Most anything can pluck it off. Here's a spectacular video of rubidium hitting water:
Naturally occurring rubidium has two isotopes: the stable 85Rb (72.2%) and the radioactive 87Rb (27.8%). 87Rb is considered only "slightly" radioactive--despite its abundance (it is naturally present in seawater)--because of its extremely long half-life of 1010 years. Remember that something has to actually decay in order for it to emit radiation.
Radioactivity is not something which has occurred naturally in the previous 36 elements except in very trace amounts (though artificial isotopes of lighter elements of course exist). Radiochemistry is a fascinating topic unto itself and becomes increasingly important for heavier elements which pretty much means from here on out.