55. Cesium

Cesium has one electron in the 6th shell. It has the same electron configuration as rubidium, but with five full shells within that have the identical configuration to xenon. Being one shell larger than rubidium, cesium has a lower ionization energy and is therefore even more reactive than the metals above it when forming the 1+ ion. Pure cesium metal reacts explosively with water and readily self-ignites in air as it reacts with (and donates its valence electron to) oxygen.

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A fun video showing cesium metal and its reactivity can be found HERE.

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OTHER GROUP I ELEMENTS: Lithium, Sodium, Potassium, Rubidium, Cesium