A chlorine atom has 17 electrons, with the outermost electron occupying a 3p subshell. In contrast, a chloride ion has gained an extra electron, resulting in 18 electrons. This additional electron occupies the 3p subshell, making it completely filled.
The outermost electrons in an atom or ion are known as valence electrons, and they significantly influence the atom's size. Valence electrons are located in orbitals, which are regions around the nucleus where the probability of finding an electron is high.
When an atom gains an electron to become an ion, the electrostatic repulsion between the negatively charged electrons increases. This repulsion causes the electron orbitals to expand and move further away from the nucleus. As a result, the overall radius of the ion increases compared to the neutral atom.
In the case of the chloride ion, the additional electron occupies the 3p subshell, which is already filled in the neutral chlorine atom. The increased electron-electron repulsion in the filled 3p subshell causes the outermost electrons to move further away from the nucleus, resulting in the larger radius of the chloride ion compared to the chlorine atom.
Therefore, the chloride ion has a larger radius than the chlorine atom due to the increased number of electrons and the resulting expansion of the electron orbitals.
