Here's why:

* Electron Configuration: Chlorine has 7 electrons in its outermost shell. It needs one more electron to achieve a stable octet configuration (8 electrons) like the noble gases.

* Electronegativity: Chlorine is a highly electronegative element, meaning it has a strong attraction for electrons.

* Ionization Energy: The ionization energy of chlorine is relatively high, indicating that it requires a lot of energy to remove an electron.

Therefore, chlorine is much more likely to gain an electron to become a negatively charged ion (Cl-) than to lose an electron and become a positively charged ion.