Chlorine has three filled or partially-filled electron orbital shells. Its electron configuration is 1s2 2s2 2p6 3s2 3p5, meaning its first and second electron orbital shells are filled, and its third electron orbital shell is partially filled.
Chlorine has an atomic number of 17, meaning that it carries seventeen electrons. Since the first electron orbital shell has a maximum capacity of two electrons and the second electron orbital shell has a maximum capacity of eight electrons, the first two electron orbital shells of a chlorine atom are filled. The capacity of the third shell is eighteen, but the third shell in a chlorine atom contains only seven electrons.