An unstable nucleus which has a tendency to spontaneously change its form with the emission of high-energy particles or photons is said to be

Radioactive

Explanation:

The radioactive nucleus is the one which does not has enough binding energy to hold the nucleus in a stable state and thus radiates either electron or proton to become a stable element.

A radioactive element is formed when after billions of years such as uranium and thorium. The stability of the nucleus depends upon the opposition of attractive and repulsive force among the nucleus.

Radioactive nucleus.

Explanation: First, we can define the radiation as the emanation of particles or photons, for example, a flashlight is radiating photons when it is turned on.

So a neutron that emanates particles or photons when change form, is a nucleus that radiates, and those are called "radioactive nucleus"

Where radioactive implies that there is a presence of radiation in some way. In this case, the radiation is caused because the nucleus goes from an energetic state to other with lower energy, and that energy must be dissipated in some way, in this case, in form of radiation.