Here's why:

* Electron configuration refers to the arrangement of electrons in the different energy levels and orbitals of an atom or ion.

* Chlorine atom (Cl): Has 17 electrons. Its electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁵.

* Chloride ion (Cl-): Gains one electron to become negatively charged. This extra electron fills the 3p orbital, resulting in the electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶.

In summary: While the number of electrons differs between the chlorine atom and chloride ion, the overall arrangement of these electrons in energy levels and orbitals remains the same.