The single-particle structure and shell gap of 100Sn is inferred from prompt in-beam and delayed γ-ray spectroscopy of seniority and spin-gap isomers. Recent results in 94, 95Ag and 98Cd stress the importance of large-scale shell model calculations employing realistic interactions for the isomerism, np-nh excitations and E2 polarisation of the 100Sn core. The strong monopole interaction of the Δl = 0 spin-flip partners πg9/2- νg7/2 in N = 51 isotones below 100Sn is echoed in the Δl = 1 πf5/2- νg9/2 pair of nucleons, which is decisive for the persistence of the N = 50 shell gap in 78Ni. This is corroborated by recent experimental data on 70, 76Ni, 78Zn. The importance of monopole driven shell evolution for the appearance of new shell closures in neutron-rich nuclei and implications for r-process abundances near the N = 82 shell is discussed.