Question

explain why most isotopes of elements with a high atomic number are radioactive??

Answer 1

As the atomic number increases, one is trying to squeeze more positively
charged protons into the limited space of the atomic nucleus. A
consequence is that heavier mass isotopes, for example, U(238) is
more stable than U(235). The additional neutrons "dilute" the
coulombic repulsion. Consistent with this model is the observation
that high atomic number elements tend to decay by emitting an alpha
particle, thus reducing the nuclear charge by two protons. Of
course, in the "real world", things are more subtle, but this is
not a bad rule of thumb.