How do the nuclei of atoms of radioactive elements differ in their structure from the nuclei

How do the nuclei of atoms of radioactive elements differ in their structure from the nuclei of atoms of ordinary elements?

The nucleus of any atom consists of a number of protons (Z) and neutrons (N).
For stable elements, the following rule is fulfilled:
N / Z = 0.98 + 0.015 * A ^ 2/3, where A = N + Z is the mass number.
For elements with heavy nuclei, N / Z – reaches a value of 1.6.
The nuclei for which this rule is not met are unstable, and the element becomes radioactive.