Which isotope is considered an analogue of potassium?

The correct answer to the question about which isotope is considered an analogue of potassium is rubidium. Rubidium shares similar chemical properties with potassium because they both belong to Group 1 of the periodic table, known as the alkali metals. This similarity in group placement means that rubidium can mimic the behavior of potassium in biological systems, similar ion charge and radii, which is important in radiopharmacy applications that involve targeting and imaging processes.

Thallium, although it may have some related properties due to being further down the periodic table in Group 13, does not exhibit the same alkali metal characteristics as potassium. Sodium, while also an alkali metal, represents a different atomic number and ionic radius compared to potassium, and calcium is not an analogue as it is an alkaline earth metal from Group 2. Therefore, rubidium is the most appropriate choice as it effectively acts as a potassium analogue in various chemical contexts.