Polarized infrared spectra and isotherms of monolayer ethene, C2H4, at 70 and 77 K on NaCl(100) are reported. Epitaxial growth of bulk ethene adlayers at 55 K on the NaCl(100) is also explored. The isotherms show that the ethene monolayer forms in constant density islands. The heat of adsorption is -20 +/- 2 kJ/mol, while the pairwise interaction energy between neighboring ethene molecules is -0.8 +/- 0.1 kJ/mol. The orientation of the ethene molecules in the monolayer is determined by polarized infrared spectroscopy. Using this orientation and making comparisons to the bulk, an ethene monolayer structure is proposed. Each unit cell in the monolayer contains two ethene molecules lying in a configuration similar to the solid with perturbations, however, due to the influence of the NaCl(100) surface. The surface density is one ethene for each sodium chloride ion pair. This proposed monolayer structure is further investigated by diperiodic group symmetry analysis and by a transition dipole-transition dipole exciton splitting calculation.