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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An alternate view (shown below) of the orbitals shows an innermost 1st-shell sphere within a 2nd tetrahedral shell (dark blue) within a 3rd cubic anti-prismatic shell (light blue) within an anti-aligned 4th cubic anti-prismatic shell (brown) within a 5th tetrahedral shell (pink) within an unpaired 6s1 electron (light green).

Cesium’s first 6 shells (left), and with the 6th shell and certain orbitals removed (right) for ease of viewing.

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