What is the electron configuration of a chloride ion?

The correct answer, which indicates the electron configuration of a chloride ion, is based on the understanding of electron configurations and the behavior of ions.

Chlorine, in its neutral state, has the atomic number 17, meaning it has 17 electrons. The electron configuration for a neutral chlorine atom is typically given as 2.8.7, reflecting that there are two electrons in the first shell, eight in the second shell, and seven in the outermost third shell.

When chlorine gains an electron to form a chloride ion (Cl⁻), it achieves a full outer shell, which is a stable electronic arrangement. The additional electron goes into the outer shell, increasing that shell's occupancy from seven to eight electrons. Thus, the electron configuration for the chloride ion becomes 2.8.8, as it now has eight electrons in the outermost shell.

This stable electron configuration is similar to that of the noble gas argon, which also has a full outer shell, contributing to the stability of the chloride ion. This full outer shell makes the chloride ion less reactive, as it has achieved a stable electronic state.