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Hydrogen is about as far as physicists can get when they try to do chemistry, at least if they stick to the traditional physics philosophy of precision. Heavier elements are just too complicated to calculate without unrealistically severe simplification of the equations (though admittedly one can do helium fairly well, given herculean effort). Even ignoring the effects of subnuclear structure, all those electrons whizz-banging about and pushing against each other make for impossible-to-solve hyper-messy nonlinear formulas.

Nonetheless, without an exact solution one can still say interesting and important things. In particular, with each step down the periodic table the nuclear charge gets one unit stronger and the innermost electrons are pulled into tighter and tighter "orbits" (to speak in classical metaphors about an intrinsically quantum phenomenon). That's why knocking an inner electron out of place in tungsten produces X-rays, photons far more energetic than the infrared or visible wavelengths that lighter elements give off in the same circumstances.

But there's a limit to that process. Keep cranking up the nuclear charge, on past uranium (atomic number Z = 92) and plutonium (Z = 94), and the innermost electrons must move faster and faster --- until, for Z greater than 137, they would have to go faster than light. That's not allowed!

Relativistic corrections to classical quantum mechanics push that limit up a bit. So does the fact that atomic nuclei aren't infinitely tiny (though they are exceedingly small compared to the "normal" size of electron orbits). But even allowing for those factors there's a hard ceiling on Z, probably in the ~150-200 area. Beyond that the electric fields near such a superheavy nucleus become so powerful that they tear the fabric of space itself apart into electron-positron pairs. The positrons are expelled and the electrons get pulled inward to neutralize the excessively positive nuclear charge. Nature doesn't abhor a vacuum, but apparently she does abhor putting too many eggs in one basket.

Or so the theoreticians think ... until somebody demonstrates otherwise. Then they will slap palms to foreheads, exclaim "How obvious!", and start publishing explanations for what they've hitherto overlooked.

(see also AppropriateUnits (2 Feb 2000), IslandsOfStability (28 Apr 2002), CoincidentalTaxonomy2 (14 May 2002), HighPrecision (16 Jul 2002), EssentialElements (4 Feb 2003), ... )

TopicScience - Datetag20031003

(correlates: TheoreticallyKnown, ClosedWindowRisks, FascinatingVersusInteresting, ...)