  Surface rippling has been observed when atoms of $X$ and $A$ are mixed on the
$A$ substrate surface. The rippling amplitude has been estimated using hard
sphere models. We present a gedanken experiment predicting a penetration of
small atoms into bulk through the (100) surface. To understand how the
electronic effects alter this picture, we investigate the surface rippling of
$X/A(100)$ from first-principles, assuming $X=$ H to Bi except for noble gases
and $A=$ Cu, Ag, and Au. We show that the small atoms (such as H, C, N, O and
F) attract electrons from the substrate due to the large electronegativity,
which prevent them from passing through the void in the (100) surface. The
behaviors of small atoms are further explored by studying lateral displacements
of the top layer in the $A$ substrate and a formation of the $X$ dimer above,
below, and across the top layer. The present work provides an example to
understand when atoms are not hard spheres.
