The process of forming a full outer shell (or achieving an octet configuration) in the case of chlorine atom (Cl) by gaining an electron to become a chloride ion (Cl-) is energetically favorable due to several factors:
Electron Configuration Stability: Chlorine has 17 electrons in its neutral state, with a configuration of 2-8-7. By gaining one electron, it achieves the stable electron configuration of 2-8-8, which is the same as the nearest noble gas, argon. Noble gas electron configurations are highly stable due to the completely filled electron shells, and this stability is energetically favorable.
Electrostatic Attraction: When a neutral chlorine atom gains an electron to become a chloride ion, the extra electron enters the outermost shell. The added negative charge of the electron creates a stronger electrostatic attraction between the nucleus (with 17 protons) and the 18 electrons. This attractive force counteracts the repulsion between the negatively charged electrons, reducing the overall energy of the system.
Lattice Energy: In ionic compounds, such as sodium chloride (NaCl) where a sodium atom donates an electron to a chlorine atom, the resulting chloride ions form a three-dimensional lattice structure due to the electrostatic attractions between the positive and negative ions. This lattice energy, which is the energy required to separate the ions in an ionic compound, contributes to the overall energy reduction when chlorine gains an electron to form Cl-.
It is important to note that the energy gain from achieving a stable electron configuration and the lattice energy generally outweigh the energy increase due to electron-electron repulsion. The precise energy values depend on various factors and can be determined through quantum mechanical calculations and experimental observations.