Author: Arnie Benn

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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

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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.

Ion formation

With a lower ionization energy than potassium, Rubidium will give up its valence electron more eagerly than potassium in an ionic interaction, in order to reach the stability of the 4s²4p⁶ noble gas configuration of krypton, which is a multi-di-electron state with four concentric full shells. That is why rubidium forms a 1+ ionic state so violently.

Pure rubidium metal can react in the presence of oxygen gas and spontaneously ignite as it donates its valence electron to oxygen. Rubidium burns with a deep-red flame, hence its name.

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

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Krypton is the sixth element with electrons in the 4p orbital. Krypton has four full shells. While the smaller noble gases are unreactive, krypton has a lower ionization energy due to its larger size to the point that it can bond with the very electronegative fluorine atom. (The wireframe indicates the boundary of the n=4 shell.)

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

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SEE OTHER NOBLE GASES: Helium, Neon, Argon, Krypton, Xenon

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Bromine is the fifth element with electrons in the 4p orbital. It has the same electron configuration as chlorine, just one shell larger. Like chlorine, bromine can support multiple possible molecular geometries, and can sustain multiply bonded atoms (with stronger electronegativity than bromine). (The wireframe indicates the boundary of the n=4 shell.)

Bonding & ion formation

Bromine is keen to obtain an extra electron to fill its fourth shell and it can bond with many atoms on the periodic table. Bromine can make one or more covalent bonds or gain an electron in ionic interaction in order to reach the stability of the 4s²4p⁶ noble gas configuration of krypton, which is a multi-di-electron state with four concentric full shells. That is why bromine forms a 1– ionic state. The negative ion is larger than its neutral version because electrons now outnumber protons. This results in a lower effective nuclear charge — a lower average attraction by the nucleus on each electron.

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OTHER GROUP VII HALOGENS: Fluorine, Chlorine, Bromine, Iodine