A model of nuclear structure in which nucleons pair up and form bosons from which collective states are formed. The IBM has had considerable success in describing the spectra of nuclei with even numbers of protons and neutrons. An extension of the model to nuclei in which there are unpaired protons or neutrons is called the interacting boson–fermion model (IBFM). Here the nuclei are analysed in terms of interaction between the unpaired fermions and the bosons made up of paired fermions.