Why is the equation of state for tin so soft?

The distribution of isoscalar monopole strength in the neutron-even Sn112-124 isotopes has been computed using a relativistic random-phase-approximation approach. The accurately-calibrated model used here (FSUGold) has been successful in reproducing both ground-state observables as well as collective excitations-including the giant monopole resonance (GMR) in Zr-90, Sm-144, and Pb-208. Yet this same model significantly overestimates the GMR energies in the Sn isotopes. It is argued that the question of Why is tin so soft? becomes an important challenge to the field and one that should be answered without sacrificing the success already achieved by several theoretical models.