Which ion in isoelectronic series has the smallest radius in crystal?

Question

Al3+

Na+

O2-

F-

N3-

Answer 1

Al^3+ would have the smallest radius in a crystal.  It has lost 3 electrons, so the nucleus would be pulling the remaining electrons in the tightest.

In general, cations (ions with a positive charge) are smaller than their isoelectronic counterparts in anions (ions with a negative charge) because the electrons already present will repel the additional electrons, making the radius larger.