  We use a scanning tunneling microscope operating in a low temperature,
ultrahigh vacuum environment to study the atomic structure of single layer
films of Cu2N grown on Cu(100). The c(2x2) lattice of Cu2N is incommensurate,
with a lattice constant of 3.72 +/- 0.02 angstrom that is 3% larger than the
bare Cu(100) surface. This finding suggests that strain due to lattice mismatch
contributes to self assembly in this system. We find that the image contrast on
Cu2N islands depends on bias voltage, which reconciles several interpretations
in the literature. We assign features in these STM images to the Cu, N and
hollow sites in the Cu2N lattice with the aid of co-adsorbed CO molecules. This
atomic registry allows us to characterize four different defects on Cu2N, which
influence the sticking coefficient and electronic coupling of adsorbates.
