In case
of head-on collision, the impact parameter
is minimum and the a-particle rebounds
back (q  @ p) For a large impact parameter,
the a-particle goes nearly undeviated and
has a small deflection (q  @ 0) The fact that only a small fraction of the
number of incident particles rebound back
indicates that the number of a-particles
undergoing head on collision is small This,
in turn, implies that the mass and positive charge of the atom is
concentrated in a small volume