The Chlorine atom has 17 electrons placed in 3 different levels.
In the outer most orbit (the one farthest from the core) Chlorine has 7 electrons. These electrons are called valence electrons.
In order to have a full valence shell, and stable configuration an atom must have 2 or 8 valence electrons.
Therefore, Chlorine atom needs 1 more additional electron in order to have a full valence shell with 8 electrons.
