Here's why:

* Valence Electrons: These are the electrons in the outermost shell of an atom, and they're the ones involved in chemical bonding.

* Octet Rule: Atoms tend to gain, lose, or share electrons to achieve a full outer shell of eight electrons (like the noble gases, which are very stable).

* Chlorine: Chlorine has 7 valence electrons. To achieve a stable octet, it needs one more electron.

This is why chlorine is highly reactive and readily forms a negative ion (Cl-) by gaining an electron.