  We predict a phase transition in freestanding monolayer Xenes from the
semiconducting phase to the excitonic insulating (EI) phase can be induced by
reducing an external electric field below some critical value which is unique
to each material. The splitting of the conduction and valence bands due to
spin-orbit coupling at non-zero electric fields leads to the formation of $A$
and $B$ excitons in the larger or smaller band gap, with correspondingly larger
or smaller binding energies. Our calculations show the coexistence of the
semiconducting phase of $A$ excitons with the EI phase of $B$ excitons for a
particular range of electric field. The dielectric environment precludes the
existence of the EI phase in supported or encapsulated monolayer Xenes.
