  We report photoemission experiments revealing the valence electron spectral
function of Mn, Fe, Co and Ni atoms on the Ag(100) surface. The series of
spectra shows splittings of higher energy features which decrease with the
filling of the 3d shell and a highly non-monotonous evolution of spectral
weight near the Fermi edge. First principles calculations demonstrate that two
manifestations of Hund's exchange $J$ are responsible for this evolution.
First, there is a monotonous reduction of the effective exchange splittings
with increasing filling of the 3d shell. Second, the amount of charge
fluctuations and, thus, the weight of quasiparticle peaks at the Fermi level
varies non-monotonously through this 3d series due to a distinct occupancy
dependence of effective charging energies $U_{\rm eff}$.
