Chlorine forms anions by gaining one electron in its outermost shell. This process is known as ionization. When chlorine gains an electron, it becomes a negatively charged ion called a chloride ion (Cl-). The ionization energy of chlorine is 1251.2 kJ/mol, which means that it takes this amount of energy to remove one electron from a chlorine atom.

The ionization of chlorine can be represented by the following equation:

Cl(g) + e- → Cl-(g)

In this equation, Cl(g) represents a chlorine atom in its gaseous state, e- represents an electron, and Cl-(g) represents a chloride ion in its gaseous state.

Chlorine atoms can gain electrons from other atoms or molecules in order to form chloride ions. For example, when chlorine gas is dissolved in water, the water molecules donate electrons to the chlorine atoms, causing them to become chloride ions. This reaction can be represented by the following equation:

Cl2(g) + 2H2O(l) → 2Cl-(aq) + 2H+(aq) + 2OH-(aq)

In this equation, Cl2(g) represents chlorine gas, H2O(l) represents liquid water, Cl-(aq) represents chloride ions in aqueous solution, H+(aq) represents hydrogen ions in aqueous solution, and OH-(aq) represents hydroxide ions in aqueous solution.

The formation of chloride ions is a common chemical reaction that occurs in many different environments. Chloride ions are found in a wide variety of compounds, including sodium chloride (NaCl), potassium chloride (KCl), and calcium chloride (CaCl2).