[en] 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 polarization of the ¹⁰⁰Sn core. The strong monopole interaction of the Δl = 0 spin-flip partners πg_9/2- νg_7/2 in N. = 51 isotones below ¹⁰⁰Sn is echoed in the Δl = 1 pf5/2- ?g9/2 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