Author: Arnie Benn

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Barium has an s-orbital di-electron in the 6^th shell. It has the same electron configuration as strontium, but with five full shells within that have the identical configuration to xenon. Being one shell larger than strontium, barium has a slightly lower ionization energy and is therefore slightly more reactive and more soluble than calcium and strontium, allowing it to form the 2⁺ ion more readily.

Barium’s outermost di-electron forms the 6^th shell, finding these valence electrons further from (and more shielded from) the charge of the nucleus. We would therefore expect this di-electron to be less well-bound than the 5^th or 4^th shell di-electrons that we find in the outermost shells of strontium and calcium. The reason is that the 6^th shell’s electron density fills a larger orbital volume. This should cause barium to be more paramagnetic — with a higher magnetic susceptibility (χ_m) value — than strontium or calcium above it on the periodic table. If electrons are further from the nucleus and less well bound, they will be more responsive to an external magnetic field. The other χ_m values on the periodic table reinforce this trend. But surprisingly, barium is less paramagnetic (with χ_m=+20) than both strontium (χ_m=+92) or calcium (χ_m=+40) above it.

This should either mean that the outer 6s² di-electron is more well-bound than the di-electrons of the 5s² and 4s² orbitals, which seems unlikely, or that something else about the electron resonance of barium gives it added stability and coherence. This might arise from barium’s unique and highly symmetrical electron configuration, depicted below.

Interestingly, this symmetry seems to create a banded shell structure in the electron cloud. It has a virtual shell of highest electron density made up of the full 3^rd and 4^th shells, with decreasing electron density as we approach either the nucleus or the outer “boundary”.

An alternate view (shown below) of the orbitals shows an innermost 1^st-shell sphere within a 2^nd tetrahedral shell (dark blue) within a 3^rd cubic shell (light blue) within a 4^th cubic shell (brown) within a 5^th tetrahedral shell (pink) within a 6s² spherical di-electron (dark green).

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

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Cesium has one electron in the 6^th 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.

An alternate view (shown below) of the orbitals 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 cubic anti-prismatic shell (brown) within a 5^th tetrahedral shell (pink) within an unpaired 6s¹ electron (light green).

Ion formation

With a lower ionization energy than rubidium, cesium will give up its valence electron even more eagerly than rubidium in an ionic interaction, in order to reach the stability of the 5s²5p⁶ noble gas configuration of xenon, which is a multi-di-electron state with five concentric full shells. That is why cesium forms a 1+ ionic state so violently.

Pure cesium metal is very soft, melting at around room temperature (~25⁰C). It is so reactive that it is considered pyrophoric, which means that it ignites spontaneously in air, and even in very cold (-116⁰C) water, as it donates its valence electron to oxygen.

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

Uses

Cesium has several industrial and manufacturing applications, but perhaps its most interesting one is its use as the element in atomic clocks that keeps time.

For an interesting video on how atomic clocks actually work, click HERE.

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

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Xenon is the sixth element with electrons in the 5p orbital. Xenon has five full shells. Like neon, argon, and krypton, it achieves symmetrical stability without the need to hybridize its 5s and 5p orbitals. It is not as unreactive as the other noble gases since its larger size means its electrons are further from and less well-bound to the nucleus. This lowers ionization energy to the point that it can bond with, for example, the electronegative oxygen and fluorine atoms. (The wireframe indicates the boundary of the n=5 shell.)

An alternate view (shown below) of the orbitals of xenon 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 cubic anti-prismatic shell (brown) within a 5^th tetrahedral shell (pink).

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

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Iodine is the fifth element with electrons in the 5p orbital. It has the same electron configuration as bromine, just one shell larger. Like chlorine and bromine, iodine can support multiple possible molecular geometries, and can sustain multiply bonded atoms (with stronger electronegativity than iodine). (The wireframe indicates the boundary of the n=5 shell.)

Bonding & ion formation

Iodine is keen to obtain an extra electron to fill its fifth shell and it can bond with many atoms on the periodic table. Iodine can make one or more covalent bonds or gain an electron in ionic interaction in order to reach the stability of the 5s²5p⁶ noble gas configuration of xenon, which is a multi-di-electron state with five concentric full shells. That is why iodine 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