Here's why:

* Chlorine's electron configuration: Chlorine has 17 electrons. Its electron configuration is 2-8-7. This means it has 7 electrons in its outermost shell.

* Octet rule: Atoms tend to gain, lose, or share electrons to achieve a stable configuration of 8 electrons in their outermost shell (the octet rule).

* Ionization: To achieve a stable octet, chlorine readily gains one electron, becoming a chloride ion. This gives it a full outer shell of 8 electrons.

* Charge: Gaining an electron gives the chloride ion a negative charge (-1).

So, a chlorine atom, when ionized, becomes a chloride ion (Cl-) with a charge of -1.