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.