Atomic and nuclear properties of positronium (Ps):
Positronium (Z = 1 atom with e+ as nucleus)
QuantityValueUnitsValueUnits
Atomic number 1 n n n
Mass* 1.097 526 752 (5) 10-3 u 1.021 991 03 (9) MeV
Mean lifetime
(annihilation)
Singlet (p-Ps): 125
(Predominately Ps→2γ)
ps Triplet (o-Ps): 142.05 ± 0.02
(Predominately Ps→3γ)
ns
* Ground state binding energy = Ry/2.

Is it reasonable to consider Ps as a chemical element?