Answer – Ions of chlorine, called chloride ions, have a charge of -1.

Explanation:

According to the octet rule, atoms ought to have 8 electrons in their valence (outermost) electron shell to become stable. However, the atoms of most elements do not have eight valence electrons. They may gain or lose electrons to achieve an octet and become electrically charged, and are then called ions. 

Atoms that require only one or two electrons to complete their valence shells can acquire them and become negatively charged, as the electrons they now possess are more than the protons in their nucleus. These negatively charged ions are called anions. 

Other atoms have only a few valence electrons (3 or lower). The atoms lose these electrons, leaving their previously penultimate electron shell, which is full, to act as the valence shell. They then acquire a positive charge as the number of protons exceeds the number of electrons. These ions are called cations. 

In general, metals become cations, while non-metals become anions. 

The electronic configuration of Chlorine is as follows.

Cl: 1s² 2s² 2p⁶ 3s² 3p⁵

Here, the valence shell is a single electron short of an octet. To achieve it, the atom has to gain an electron. 

Cl + e^– → Cl^–

The electronic configuration now is,

Cl: 1s² 2s² 2p⁶ 3s² 3p⁶

Cl^– is called chloride, and is an anion. It has a charge of -1 as the ion now has one electron more than its protons.