Effects of the open-shell electronic structure of atomic Sn on the $4d$ photoelectron and Auger electron spectra have been studied by both theory and experiment. The $5p$ and $4d$ photoelectron spectra and the ${4d}^{\ensuremath{-}1}\ensuremath{\rightarrow}{5p}^{\ensuremath{-}2}$ Auger spectrum have been measured from atomic tin using synchrotron radiation for excitation. Relativistic Dirac-Fock calculations have been carried out for the energies and intensities of photoelectron and Auger electron transitions from each thermally populated initial state level. It has been shown by a comparison of the experimental and theoretical spectra that the weakly populated excited neutral states, negligible in the direct photoionization process, play an important role in the formation of the Auger spectrum of the open-shell atoms.