What is the electron configuration of the chloride ion Cl−, and why does it resemble argon?

• A. Cl− = [Ne] 3s2 3p5; lacks noble-gas symmetry.
• B. Cl− = [Ne] 3s2 3p6; full valence shell like Ar.
• C. Cl− = [Ar] 3d10 4s2; incorrect inner shells.
• D. Cl− = [Ar] 4s2 3d10; incorrect.

Answer: (B)

Gaining electrons to complete the outer shell creates a stable, noble-gas–like arrangement. Chlorine’s neutral configuration is [Ne] 3s2 3p5, so by adding one electron it becomes [Ne] 3s2 3p6. That fills the 3p subshell and gives a total of 18 electrons—the same as argon, which has [Ne] 3s2 3p6 as its outer shell. So Cl− is effectively isoelectronic with argon, sharing the same electron count and a full valence shell.

The other descriptions would place electrons in a way that doesn’t reflect chlorine’s actual electron arrangement: one shows the neutral form with 3p5, not a full octet; the others imply inner-shell configurations (like 3d10 or misordered 4s) that don’t correspond to chlorine’s energy levels.