  A structural model for intermediate sized silicon clusters is proposed that
is able to generate unique structures without any dangling bonds. This
structural model consists of bulk-like core of five atoms surrounded by
fullerene-like surface. Reconstruction of the ideal fullerene geometry results
in the formation of crown atoms surrounded by $\pi$-bonded dimer pairs. This
model yields unique structures for \Si{33}, \Si{39}, and \Si{45} clusters
without any dangling bonds and hence explains why these clusters are least
reactive towards chemisorption of ammonia, methanol, ethylene, and water. This
model is also consistent with the experimental finding that silicon clusters
undergo a transition from prolate to spherical shapes at \Si{27}. Finally,
reagent specific chemisorption reactivities observed experimentally is
explained based on the electronic structures of the reagents.
