An example of a radioactive isotope is
carbon-12.
carbon-14.
neon-20.
neon-22.

Carbon-12 is not an isotope because carbon has an atomic number of 12. Although neon-20 and neon-22 are isotopes of neon, which has an atomic number of 10, they are considered to be 2 of the 3 stable isotopes of neon (the other being neon-21). This leaves carbon-14 which is both radioactive and an isotope of carbon.

Organisms are more likely to be more preserved in mud than sand because mud is moist and wet, while sand is very dry.