Barium has an s-orbital di-electron in the 6th 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, it 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 has a full 5th shell within it, now featuring two full d-orbitals and a remarkable electron orbital symmetry (see below). While there are again more electrons shielding the charge of the nucleus, and barium’s outer electrons would seem less strongly bound than in strontium, barium is nevertheless significantly less paramagnetic (with χm=+20) than strontium (χm=+92), and even than calcium (χm=+40). This would either mean that the outer 6s2 di-electron is more well-bound, as a di-electron, than are the di-electrons of the 5s2 and 4s2 orbitals, which seems unlikely, or else it suggests that the electron symmetry of barium, depicted below, may result in a more coherent harmonic electron state than the elements above it on the periodic table can achieve.
An alternate view of the orbitals shows an innermost sphere within a 2nd shell tetrahedral (dark blue) within a 3rd shell octahedral (light blue) within a 4th shell octahedral (brown) within a 5th shell tetrahedral (pink). (The 6s2 shell is omitted for visual clarity.)
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