Adsorption and conversion of ethylene to ethylidyne on flat (111) and stepped Pd surfaces have been studied with the aim to unravel the complex chemistry of small organic molecules on Pd. These processes are crucial to understanding many experimental observations on Pd catalysts involved in selective hydrogenations, steam reforming, polymerization, and several other chemical processes. Our results provide a view on the complex chemistry of olefins on the surface, where several competitive processes take place simultaneously and where a hierarchy among different bond activations can be established. For Pd, the C-H bonds of the olefins are the most labile on the surface, followed by C-C and last isomerization processes. From the picture above not always the most straightforward reaction mechanism is necessarily the one taking place on the surface. Scrambling of H atoms on the organic moieties is the most effective way to generate certain (even long lasting) isomers on the surfaces.