Rubidium has one electron in the 5th shell. It has the same electron configuration as potassium, but with four full shells within that have the identical configuration to krypton. Being one shell larger than potassium, rubidium has a lower ionization energy and is therefore more reactive, forming the 1+ ion more readily. Pure rubidium metal will self-ignite in air as it reacts with (and donates its valence electron to) oxygen.
A fun video showing rubidium metal and its reactivity can be found HERE.
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