Question

What eventually happens to radioactive isotopes?

Answer 1

They will decay to another nuclide.

Explanation:

First of all it is important to know the difference between isotope and nuclide, for these terms are often confused:

• isotope: a variety of an element with the same number of protons and therefore the same chemical properties. Isotopes differ in the number of neutrons or energy in the nucleus
• nuclide: a more general term that refers to varieties of elements that are not necessarily isotopes.

Radioactive decay usually changes the proton number in the nucleus and therefore one decay of 'isotopes' is not the correct term, it is decay of nuclides.

Unless you talk about gamma decay (`gamma`) or internal conversion, in that case one isotope has an excess energy (metastable state), when it decays it releases this excess energy to become the stable version of the same isotope.

Explanation with an example
Technetium-99m (metastable) decays to Technetium-99, this is the same nuclide but can be considered an `color(red)"isotope"`:

`"^(99m)Tc` `-> "^99Tc + gamma`

When this `"^99Tc` would have been a stable nuclide it would end there, but it continues to decay by emitting `beta^-`particles (neutron converted to a proton):

`"^99Tc` `-> "^99Ru + beta^-`

So it decays to Ruthenium which is a different `color(red)"nuclide"`. This Ru-99 is stable and that is where decay ends. Eventually all unstable nuclides will decay to a stable nuclide, this can take multiple steps and millions of years.