The radioactive isotope Sn was studied using Coulomb excitation at the REX-ISOLDE facility at CERN. This is the lightest odd-Sn nucleus examined using this technique. The reduced transition probability of the lowest-lying 3/2 state was measured and is compared to shell-model predictions based on several sets of single-neutron energies relative to Sn. Similar to the transition probabilities for the 2 states in the neutron-deficient even-even Sn nuclei, the measured value is underestimated by shell- model calculations. Part of the strength may be recovered by considering the ordering of the d and g single-neutron states.