Scanning tunneling microscopy and molecular beam techniques were used to investigate the chemisorption sites of O-2 onto Si(111)-7x7 surfaces as a function of O-2 incident energy (E(i)). It is shown that there are two different O-containing sites being formed, i.e., bright and dark sites. The distribution of dark sites is not sensitive to E(i). However, the distribution of bright sites is a strong function of O-2 incident energy. At E(i)=0.02 eV, there is no preference between bright corner-adatom and center-adatom sites (about 1.1:1) while at E(i)>0.06 eV, the bright sites are preferentially located at corner-adatom sites over center-adatom sites by about 2:1. It is proposed that difference in the spatial distribution of adsorbates at different incident energies results from different chemisorption mechanisms : precursor-mediated (physisorption) chemisorption at low incident energy and direct activated chemisorption at high incident energies.