Here's why:

* Electron Configuration: Chlorine's electron configuration is 1s²2s²2p⁶3s²3p⁵.

* Valence Shell: The valence shell is the outermost shell of an atom, which is the 3rd shell in chlorine's case.

* Valence Electrons: The electrons in the valence shell are called valence electrons. Chlorine has 2 electrons in the 3s subshell and 5 electrons in the 3p subshell, totaling 7 valence electrons.