38. Strontium

Strontium has an s-orbital di-electron in the 5th shell. It has the same electron configuration as calcium, but with four full shells within that have the identical configuration to krypton. Being one shell larger than calcium, its valence electrons are less well bound. This causes it to have a lower ionization energy and be more reactive and more soluble than calcium, allowing it to form the 2+ ion more readily.

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OTHER GROUP II ELEMENTS: Beryllium, Magnesium, Calcium, Strontium, Barium

37. Rubidium

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.

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

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