Here's why:

* Chlorine's Atomic Structure: Chlorine has 17 electrons. Its outermost shell (the valence shell) has 7 electrons.

* Octet Rule: Atoms tend to gain, lose, or share electrons to achieve a stable configuration of 8 electrons in their outermost shell.

* Gaining an Electron: To achieve a full octet, chlorine gains one electron. This gives it a total of 18 electrons, with 8 in its outermost shell.

* Negative Charge: By gaining an electron, chlorine gains a negative charge, as electrons carry a negative charge.

Therefore, chlorine gains one electron to form a chloride ion (Cl-) with a single negative charge.