This uses similar logic to atomic radius, but we will be looking at comparing the sizes of ions.
Gaining or Losing Electrons
Gaining Electrons: When an atom/ion gains electrons, the electron-electron repulsion increases. This pushes the electrons further away, increasing the ionic radius.
Ex. Cl⁻ is larger than Cl because Cl⁻ has an extra electron.
Losing Electrons: When an atom/ion loses electrons, the electron-electron repulsion decreases. The electrons are closer to the nucleus, so ionic radius decreases. Often, ions will lose entire energy levels, which largely decreases the ionic radius.
Ex. Na is larger than Na⁺. Na⁺ completely lost its third principal energy level, so it became much smaller than sodium.
Isoelectronic Species
When ions are isoelectronic, look at the amount of protons. The more protons, the more the electrons are attracted to the nucleus, and the smaller the ion.
Ex. Ca²⁺, Ar, and Cl⁻ all have 18 electrons. Ca²⁺ is the smallest with 20 protons. Argon is in the middle with 18 protons. Cl⁻ is the largest with 17 protons.
Chemistutor 