What is the valency of Chlorine?
Cl has an atomic number of 17. Thus, it should have 7 e− in its outermost shell.
7 of them are shown in the valence shell of Lewis structure.
The valence shell is the outermost shell (that includes electrons) in terms of the shell energy. The kernel electrons are the electrons in the shells "inside" the valence shell.
∴ The kernel should have 10 e−s.