Either two atoms or three atomsThe energies of the photons emitted can be expressed as follows:13.6(121​−221​)eV=10.2eV 13.6(121​−321​)eV=12.1eV 13.6(221​−321​)eV=1.9eV The following table gives the transition corresponding to the energy of the photon:A hydrogen atom consists of only one electron. An electron can have transitions, like from n=3 to n=2 or from n=2 to n=1, at a time.So, it can be concluded that the photons are emitted either from three atoms (when all the three transitions of electrons are in different atoms) or from two atoms (when an atom has n=3 to n=2 and then n=2 to n=1 electronic transition and the other has n=3 to n=1 electronic transition).