Question

What do you mean by stability of an atom

Answer 1

A stable element by definition is a chemical element (found on the Periodic Table) that has atleast one stable, naturally occurring isotope. For example, Helium has 9 isotopes, but since two of them are stable (He-3 and He-4), Helium is considered to be a stable element. By extension, an unstable element is a chemical element that does not have a single stable, naturally occurring isotope. For example, technetium is the lightest element on Earth with no stable isotope (all of its isotopes are radioactive!) So basically when an atom is said to be stable it will be having completly filled orbital or will be having octet electron in last shell.