This paper reports theoretical and experimental studies that explain how the size-dependent shape and elemental composition of PbS quantum dots (QDs) affect the arrangement and density of adsorbed ligands by QDs surfaces, and their related susceptibility to oxidation. By considering the differences of Pb:S ratios of the particles, we predict a size-dependent Pb:S ratio that is consistent with our measurements, and those reported in the literature. We further connect the shape and facet Pb:S composition to surface ligand densities, and relate this to the particle-size dependence of oxidation when exposed to air. While the surfaces of smaller particles (d < 2.7 nm) are entirely covered by Pb terminated (1 1 1) facets, bigger QDs (2.7 < d < 7.5 nm) are truncated at their (1 00) facets to form cuboctahedron-shape particles. The (1 1 1) facets representing the largest portion of both smaller and bigger particle's surfaces can be well-passivated by anionic oleate/hydroxide ligands. Investigation on the surface chemistry of PbS QDs by the means of FT-IR, NMR, XPS and combustion elemental analyses, as well as steric stability considerations suggested that hydroxide to oleate atomic ratio in (1 1 1) facet of PbS QDs is about 1:5. On the other side, (1 0 0) surface facets have a checkerboard arrangement of lead and sulfur atoms and are coordinated by oleic acid ligands. Low density and weak binding energy of such ligands make the (1 0 0) facets vulnerable to oxidation in an air atmosphere. XPS investigations indicate that the oxide includes both PbSO3 and PbSO4 components on the surface of PbS QDs.