A stable electron configuration refers to an atom in which the outer electron shell is full.
Outermost electron shells hold a maximum of 2, 8, 8, 18,...(etc.) electrons.
With an outer shell of 2 electrons, the periodic table shows that helium has a stable electron configuration.
The next stable configuration of an element has 10 (2 + 8) electrons. This is neon.
Neon is followed by argon, with 18 (2 + 8 + 8) electrons.
Krypton, with 36 (2 + 8 + 8 + 18) electrons is next.
The remaining stable configurations follow a similar pattern, based on the maximum number of electrons able to fit into the outermost ring. These elements are extremely stable and rarely react with other elements. They are referred to as the noble gases or inert elements.
Atoms of other elements may bond with each other or different elements to form molecules having full outermost shells of electrons.