When a chlorine atom becomes chloride ion, its size — Periodic Table and Periodicity Chemistry Question

💡 Solution & Explanation

# Chlorine Atom to Chloride Ion: Size Change **Step 1: Electron Configuration Change** - Chlorine atom ($Cl$): $1s^2 2s^2 2p^6 3s^2 3p^5$ (17 electrons) - Chloride ion ($Cl^-$): $1s^2 2s^2 2p^6 3s^2 3p^6$ (18 electrons) **Step 2: Orbital Occupancy** When $Cl$ gains one electron to form $Cl^-$, the 3p subshell becomes completely filled. Both have the same nuclear charge (17 protons), but the ion has one additional electron. **Step 3: Electron-Nuclear Attraction** - More electrons repel each other → increased electron-electron repulsion - The same nuclear charge now attracts more electrons - The net effect: electrons are **less tightly pulled** toward the nucleus **Step 4: Effective Nuclear Charge** The effective nuclear charge decreases because the additional electron increases shielding. Outer electrons experience less net attraction. **Result:** The ionic radius of $Cl^-$ is **significantly larger** than the atomic radius of $Cl$. $$r(Cl^-) > r(Cl)$$ **Why B is correct:** The size **increases** when chlorine becomes chloride ion due to increased electron-electron repulsion and decreased effective nuclear charge, despite constant proton number.