Cobalt-60 is radioactive despite having a neutron-to-proton (n/p) ratio of 1.22, which is below the commonly referenced threshold of 1.56 for radioactivity. This is explained by the Mattauch rule, which states that among isotopes with the same mass and differing by one proton, at least one must be radioactive. Cobalt-60 is classified as an odd-odd isotope, which typically leads to instability and radioactivity, unlike the few exceptions like Tantalum-180. The discussion highlights that the n/p ratio is not the sole determinant of radioactivity, as energetic favorability plays a significant role. Thus, Cobalt-60's radioactivity is consistent with its classification under these nuclear stability principles.