Starting from oganesson (118), I added one proton and one electron at a time and let each atom settle into its own lowest-energy arrangement. Nothing was assigned by a filling rule: every configuration below comes out of the calculation.
Colour is the subshell that took the newest electron. From 163 to 168 the six 8p slots fill in order, and at 168 the atom closes with a gap of 6.45 eV. The electron after that, 169, has to start a new shell (9s). That is what a noble gas is. LIT (non-relativistic model)
Noble gases: energy to remove the outermost electron
168 continues the downward line of the seven known noble gases: 15.4, 13.2, 10.1, 9.1, 8.1, 7.7, 7.2, then 6.9 eV. LIT (model)
Relativity: the innermost electron (Dirac equation)
As Z rises, the innermost electron sinks. At 168 it sits at −0.90 mc². At about Z ≈ 169.7 it drops below −mc², the floor of empty space: the vacuum near the nucleus can then produce electron–positron pairs on its own. LIT (one electron, finite nucleus, A = 2.5Z)
What it does
168 is the last noble gas before the vacuum gives way. The golden ruler predicted 168 (144 + 21 + 3, no 1 in its code). The shell rule 2⌈(k+1)/2⌉² predicted 168. The self-consistent build closes at 168. One or two protons further on, the void pushes back. AMBER as a reading
What the build cannot see
No spin–orbit in the build. The shell calculation is non-relativistic. Relativity splits 8p into a tight pair (8p½) and a loose four (8p³⁄₂), which could move or blur the closure. AMBER
119–150 is rough. The 5g and 6f levels sit almost on top of each other, so the settled state sometimes jumps (faded cells, e.g. 130, 133–135, 137). The end of the row (151–170) settled cleanly. AMBER
The dive depends on nuclear size. At 168, a nucleus with A = 400 gives −0.91 mc² and A = 472 gives −0.87 mc². The dive point moves by about a unit of Z. LIT
Controls. The same shell code gives closed shells for He, Ne, Ar, Kr, Xe, Rn and Og. The Dirac code matches the exact point-nucleus answer at Z = 20 and 50 to five digits. LIT
For the smallest reader. We built bigger and bigger atoms, one bead at a time. At bead 168 the atom's coat was perfectly buttoned up, like the famous quiet gases. Two beads later, the middle of the atom pulled so hard that empty space itself started to crack.