An imbalance between the electrostatic and strong nuclear forces
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
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)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.
The two aspects that cause the nucleus of any element atom to be unstable are:not have the specific neutron/proton ratio to be a stable nucleus, and orhaving number of protons that exceeds the stability limit (exceeding 83).Referring to question below for more information.
Increasing the ratio neutrons/protons in the nucleus the atom become unstable.
Decay
It isn't really an ELEMENT that is unstable, but an ISOTOPE. That means that in general, for the same element, some atoms will decay, and some will not - the difference being the number of neutrons in the nucleus.
Magnesium
Too many or too few neutrons.
The nucleus would become unstable because you need a certain amount of neutrons, electrons, and protons for it to be stable.
The two aspects that cause the nucleus of any element atom to be unstable are:not have the specific neutron/proton ratio to be a stable nucleus, and orhaving number of protons that exceeds the stability limit (exceeding 83).Referring to question below for more information.
the atom to become unstable and rip apart
Increasing the ratio neutrons/protons in the nucleus the atom become unstable.
the unstable nucleus will decay into smaller, stable particles.
Decay
A stable nucleus is one which will not decay, whereas an unstable nucleus will decay at some point, which cannot be predicted as decay is a random process, by alpha or beta decay.
Magnesium
It isn't really an ELEMENT that is unstable, but an ISOTOPE. That means that in general, for the same element, some atoms will decay, and some will not - the difference being the number of neutrons in the nucleus.
radioisotope
the unstable nucleus of an atom