This assumes that the atom in question is a hard sphere, the radius of which varies on its ionization state. This is a gross simplification to prime the readers mind. Typically, a cation has a smaller ionic radius because of the loss of an electron and an anion has a a larger ionic radius because of the addition of an electron.
Feel free to be the first
Please review the linked page for context.
If you can think of something better than this,
please add it to the database