  The variances of the fission fragment mass distributions for symmetric
fission over a wide range of the fissility of the compound nucleus have been
investigated within the frame work of Random Neck Rupture Model (RNRM) proposed
by Brosa et. al.,. The shape of the fissioning nucleus is generated using
crel=1 in the RNRM model, which results in a smooth and continuous shape of
fissioning nucleus. This modified shape of RNRM model has been used to analyse
experimental data of mass variances for symmetric mass distributions of 27
systems leading to understanding of scission shape dynamics in a wide region of
fissility 0.7-0.95. The neck radius is an important parameter of the RNRM model
and this has been varied to fit the experimental mass variances data. The
systematics of resulting neck radii are studied of as a function fissility and
nuclear potential through {\gamma}o, the surface energy coefficient. Further
average total kinetic energies of fission fragments <TKE> are studied of as a
function of fissility and {\gamma}o parameter. It is observed that the neck
radii that fit the experimentally observed variances of the mass distributions
fall into two groups and these groups are related to two groups of
experimentally observed <TKE>s. Empirical formulae have been obtained for the
neck radii for these two groups of fissioning systems. Use of the empirical
formulae for neck radii predict the mass variances reasonably well for these
systems and it is shown that these empirical neck radii are better estimates
than starting with the Rayleigh criterion.16pages,7 figures
