🚀 go-pugleaf

Is positronium an atom?  Yes and no.  It is sometimes called an "exotic
atom".  It doesn't have a nucleus in the usual sense of
       i.)  A massive 'core' around which the electron(s) are located
            (because both the positron and electon are distributed
             around their 'geometric midpoint'...[quantum equivalent
             that is]
      ii.) The 'nucleus' does not consist of nucleons.

Are there other such misfits that don't get placed on the Periodic
Table?  Plenty.  An anti-atom won't be there, such as an
antiproton+antielectron=antihydrogen atom.  There are also muonium
'atoms' with a (positive) muon and an electron, and other combinations
as well.  The Periodic Table (in any of its hundred odd forms) is
supposed to be a visual guide to chemical (electronic) behavior.  These
exotic species have chemical behavior that's interesting, but scientists
working with them don't need a table of all the usual elements to remind
them of how it all fits together.  And the usual readers of the Periodic
Table are rarely interested in atoms outside the conventional collection.

Paul J. Karol
Nuclear Chemist
(and occassional positronium user)