Atomic and nuclear properties of materials:
Positonium (atom with e+ as nucleus)
Quantity
Value
Units
Value
Units
Atomic number
1
n
n
n
Mass*
+/- 1.097 526 752
+/- 0.000 000 005
10-9 u
+/- 1.021 991 03
+/- 0.000 000 09
MeV
Mean lifetime (annihilation)
Singlet: 125
ps
Doublet: 140
ns
* Ground state binding energy = Ry/2.
Is it reasonable to consider Ps as a chemical element?
It is bound electronically. If we consider atoms as having orbital
electrons, then the positron plays the role of nucleus.
It has a rich chemistry, as Google searches on "positronium halides,"
"positronium compounds," and other strings show. There are international
conferences on positronium chemistry.
In other exotic atoms such as muonium, pionium, and kaonium, the bound
muon or meson plays the role of an orbital electron instead of being the
nucleus. Although e-pi+ and other such atoms might equally well
be regarded as elements, as would atoms with hypernuclei, these
contain
unstable particles. An isolated positron is presumably as stable
as an electron.
One serious objection is that the positronium "nucleus" has non-zero
lepton number.
(e-pi+ and atoms with hypernuclei have nuclei with lepton number,
zero, as do "normal" nuclei.
We are physicists, not chemists.
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