Name the cations that are largest single atom cations.
Neutral atoms that have lost an electron exhibit a positive charge and are called cations. These cations are smaller than their respective atoms; this is because when an electron is lost, electron-electron repulsion decreases and the protons are better able to pull the remaining electrons towards the nucleus. A second lost electron further reduces the radius of the ion. For instance, the ionic radius of Fe2+ is 76 pm, while that of Fe3+ is 65 pm. If creation of an ion involves completely emptying an outer shell, then the decrease in radius is especially great.
Neutral atoms that have gained an electron are called anions, and they are much larger than their respective atoms. As an additional electron occupies an outer orbital, there is increased electron-electron repulsion which pushes the electrons further apart. Because the electrons now outnumber the protons in the ion, the protons can not pull the extra electrons as tightly toward the nucleus.