When is an element stable?

Answer 1

For an element to be stable, it must have a full outer shell (valence shell) of electrons. An element with a full outer shell of electrons will act like its nearest noble gas and be stable. Most elements require 8 valence electrons to be stable, however H needs 2.

Answer 2

An element is stable when its nucleus is not undergoing radioactive decay, meaning it has a balanced ratio of protons and neutrons. Stable elements typically have relatively low atomic numbers, while elements with higher atomic numbers tend to be unstable and decay over time.