We have observed the Si(1 1 1) 7 x 7 surface slightly exposed to 0.03 L (1 x 10(-9) Torr, 30 s) oxygen molecules at room temperature by using home-built noncontact atomic force microscopy (NC-AFM) with true atomic resolution. When the tip was far enough of the surface to sample distance, bright spots which corresponded to oxygen were observed, although the Si(111) 7 x 7 atomic structure did not appear. As the tip became closer to the sample, the Si(111) 7 x 7 atomic structure starts to appear gradually with oxygen bright spots simultaneously. For large tip-sample distance, the interaction between the tip and the sample is mainly mediated by long-range forces such as electrostatic or van der Waals forces. In this region, NGAFM does not provide the atomically resolved Si(111) 7 x 7 image. On the contrary, at closer tip-sample distance, NGAFM provides atomically resolved 7 x 7 images due to short-range interaction such as chemical bonding force. (C) 2003 Elsevier Science B.V. All rights reserved.