A study of the adsorption of ethylene C2H4 on Si(111)7 X 7 at room temperature by scanning tunneling microscopy and scanning tunneling spectroscopy is reported. No significant change in the surface reconstruction is observed. The spatial distribution of the surface reaction has been analyzed on an atom-by-atom basis. Reacted and unreacted sites have been selectively imaged. A higher reactivity of the adatoms in the faulted subunits with respect to those in the unfaulted subunits is reported. Furthermore, the center adatoms are more reactive than the corner adatoms. On the basis of this distribution of reacted atoms, the adsorption site is assigned. The proposed bond model implies that an ethylene molecule is bonded to a pair of adjacent Si adatom-rest atom. It is also shown that the adsorption process affects not only the surface atoms directly involved in the chemical bond with the ethylene molecules, but also some of the neighboring adatoms.