What would cause an nucleus to be unstable?

Answer 1

An imbalance between the electrostatic and strong nuclear forces

Answer 2

Short Answer:

Stable isotopes of elements have a Strong force that completely overwhelms the repulsive static charge of protons. (Stable isotopes also have a lower ratio of neutrons/protons than unstable isotopes.)

Instability occurs when there is a Close Balancebetween the binding effect of the Strong force and the electrostatic force repelling protons. Less energy is required to disrupt this near-balanced state.

There is also a way for a nucleon to momentarilychange identity between that of a proton and that of a neutron (or vice-versa). This occurs through the weak force (which has a property of symmetry-breaking), and requires the simultaneous presence of a relatively rare subatomic particle and a rather specific amount of energy. This is the mechanism behind spontaneous fission, AKA radioactive decay.

An unstable nuclei cannot survive this identity "crises" even momentary. The quantum state of the nuclei is not "allowed". An imbalance between the electrostatic and strong nuclear forces

Answer 3

An unstable nuclide can attain stability through one or series of nuclear transformations until the proton-neutron ratio in the nuclide changes to the ratio corresponding to stability or until the number of protons in the nuclide decreases to the stable proton number limit.

(see related questions below for more information)

Answer 4

A nuclide is determined to be stable if it does not spontaneously undergo radioactive decay, and is not radioactive itself. There are presently 80 stable nuclides or isotopes.