The reactivity of atoms and ions is determined by their tendency to gain, lose, or share electrons to achieve a more stable electron configuration. While it is true that atoms generally strive to have a full valence shell of electrons (typically 8 electrons for most elements), there are some important factors to consider:
Ionization energy: The energy required to remove an electron from an atom is known as ionization energy. In the case of chlorine, it has 7 valence electrons in its neutral atom, and removing one electron to form a chloride ion (Cl-) requires overcoming a relatively low ionization energy. Thus, chlorine has a greater tendency to lose one electron and achieve a stable octet configuration, resulting in the more stable chloride ion.
Electron affinity: Electron affinity is the energy change that occurs when an atom gains an electron. Chlorine has a high electron affinity, meaning it has a strong attraction for an additional electron to complete its octet. By gaining one electron, chlorine can achieve a stable electron configuration like its neighboring noble gas, argon. This additional electron fills the 3p orbital, resulting in the more stable chloride ion.
Increased stability through ionic bonding: When chlorine atom gains an electron and forms a chloride ion, it becomes negatively charged. This allows it to interact with other atoms or ions through ionic bonding, which leads to the formation of ionic compounds. In ionic compounds, the ions are held together by electrostatic attractions, resulting in a more stable overall structure.
So, while it may seem counterintuitive that adding an electron to an atom would make it more unstable, in the case of chlorine, the formation of a chloride ion leads to a more stable configuration and greater reactivity in terms of its ability to form compounds with other elements.