# Why are there no particles with charge, but no isospin?

The standard model has two parallel series of leptons: the charged leptons (electron, muon, and tau) with -1 electric charge and -1/2 weak isospin, and the neutrinos, with 0 electric charge and +1/2 weak isospin.

Why do we not have a third series of particles with non-zero electric charge, but no isospin? I.e., particles with purely electromagnetic interactions, and no weak interaction? Does something else horribly break if such fields are added to the standard model?