standard model - If mesons were stable could they form atoms?

if there were stable enough mesons similar to protons and neutrons could they capture electrons to form exotic elements.

if not why is this not possible?

Answer

As anna mentioned, there are non quark models which clarify exotic hadrons. In principle, they are allowed in Quantum Chromodynamics (QCD). Non-quark models predict

1.hybrid mesons: Include quark anti-quark pair and gluon.

2.Glueballs: Gluons are their own bound states.

3.Exotic hadrons as in figure below

which exchange pion at low energies (couple of GeV scale) and can form a new bound state of multi-quarks as a $DD$ molecule. $D$ meson is one of the lightest meson which is very unstable check pdg list, indeed. It's mean life time about ~$10^{-15} s$.

Above these thresholds, there are some expected exotic hadrons which do not contribute quark model.

Big experiments such as LCHb, CMS at LHC, are trying to solve this puzzle. And so far, they observed X(3872), Z(4430), Y(4140) which are candidates of exotic hadrons. They did not fit into the quark model because they have unexpected narrow width and also unlikely branching ratios . These new particles are expected to have a non-quark form such as multi-quark exotic hadrons. The analysis of these particles still ongoing.