How can a molecule that contains an atom of phosphorus be made radioactive using an unstable isotope?

A radioactive (unstable) phosphorus isotope is made to replace a nonradioactive isotope of phosphorus in the molecule

Explanation:

Phosphorus has both unstable (radioactive) and stable isotopes. In any compound that contains phosphorus, the phosphorus atom present may be either a stable isotope of phosphorus or an unstable isotope of phosphorus.

However, if the molecule contains a stable nonradioactive isotope of phosphorus, it is possible to convert this isotope in the molecule to the unstable radioactive isotope by carefully carrying out exchange reactions in which the phosphorus isotope replaces the nonradioactive isotope in the molecule.

This is usually done in order to create radioactive tracers which are used to study chemical reactions and biochemical processes.