Chlorine has 7 electrons in the third ring, or third electron shell, of its atom.
A neutral chlorine atom has atomic number 17, meaning it contains 17 electrons. These are arranged in shells as 2, 8, and 7. The first shell holds two electrons, the second holds eight, and the remaining seven occupy the third shell. In orbital notation, chlorine’s outer electrons are represented by 3s² 3p⁵.
The seven electrons in chlorine’s outer shell explain much of its chemistry. Chlorine needs one more electron to complete a stable outer shell, so it commonly gains an electron and forms the chloride ion, Cl⁻. This is also why chlorine readily combines with sodium: sodium loses one electron, while chlorine gains it, producing sodium chloride, or table salt.
The third shell is sometimes called the M shell. A common mistake is to answer 17, which is chlorine’s total number of electrons, or 5, which is the number of p-orbital electrons in its outer subshell. The question asks specifically for the third shell, so the canonical answer is 7.