* Same mass number (40): They all have 40 nucleons (protons and neutrons) in their nucleus.

* Isobars: They are isotopes of different elements that have the same mass number.

* Involved in radioactive decay: Potassium-40 is a radioactive isotope that can decay into both Argon-40 and Calcium-40.

Here's a breakdown of their differences:

* Number of protons:

* Argon-40 (Ar-40): 18 protons

* Potassium-40 (K-40): 19 protons

* Calcium-40 (Ca-40): 20 protons

* Number of neutrons:

* Ar-40: 22 neutrons

* K-40: 21 neutrons

* Ca-40: 20 neutrons

* Stability:

* Ar-40: Stable isotope

* K-40: Radioactive isotope

* Ca-40: Stable isotope

Let me know if you'd like more detail on the radioactive decay of Potassium-40!