Question

what differs between a radioactive isotope and a stable isotope of the same element?
radioactive isotopes last for less time.
stable isotopes are usually heavier.
radioactive isotopes do not break down.
stable isotopes break down quickly.

Explanation:

Isotopes are atoms of the same element with different numbers of neutrons. Radioactive isotopes are unstable and decay over time, emitting radiation, so they have a limited lifespan. Stable isotopes do not decay spontaneously and persist indefinitely. Heavier isotopes are not necessarily stable. Radioactive isotopes break - down and stable isotopes do not break down quickly.

Answer:

Radioactive isotopes last for less time.