Potassium has the same electron configuration as sodium, but with three full shells within that have the identical configuration to argon. Being one shell larger than sodium, potassium has a lower ionization energy and is therefore more reactive.
Potassium will give up its valence electron eagerly in an ionic interaction in order to reach the stability of the 3s23p6 noble gas configuration of argon, which is a multi-di-electron state with three concentric full shells. That is why potassium forms a 1+ ionic state so violently.
Pure potassium metal reacts explosively when placed in water as it donates its valence electron to oxygen. The heat of this reaction ignites the hydrogen gas that is also produced from the water. Potassium burns with a purple flame and throws purple sparks. It is used to create this same effect and color in fireworks. A fun video showing potassium metal and its reactivity can be found HERE.
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