* Electrons and Size: The size of an atom or ion is primarily determined by the number of electron shells and the attraction between the electrons and the nucleus.

* Chlorine Atom: A chlorine atom has 17 electrons. These electrons are distributed in three electron shells.

* Chlorine Ion: When a chlorine atom gains an electron to become a chloride ion (Cl-), it has 18 electrons. The extra electron fills the outermost shell, increasing the electron-electron repulsion. This repulsion pushes the electron shells farther apart, making the ion larger than the atom.

In summary, the added electron in the chlorine ion leads to increased electron-electron repulsion, causing the ion to be larger than the neutral atom.