Chlorine needs to gain one electron to have a full outer shell.

Here's why:

* Electron Configuration: Chlorine has 17 electrons. Its electron configuration is 2-8-7. This means it has 7 electrons in its outermost shell (the third shell).

* Octet Rule: The octet rule states that atoms tend to gain, lose, or share electrons to achieve a stable configuration of eight electrons in their outermost shell.

* Gaining an Electron: By gaining one electron, chlorine completes its outer shell, achieving a stable electron configuration of 2-8-8.