56. Barium

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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