Order the following ions from smallest to largest: Mg2+, O2-, F-, Na+.

Ionic radius refers to the size of an ion in a crystal lattice. Cations, like Mg2+, are smaller than their neutral atoms due to the loss of electrons, which reduces electron-electron repulsion and allows the remaining electrons to be pulled closer to the nucleus. Conversely, anions, such as O2- and F-, are larger than their neutral counterparts because the addition of electrons increases repulsion among them, leading to a larger radius.

Effective nuclear charge (Z_eff) is the net positive charge experienced by an electron in a multi-electron atom. It accounts for the shielding effect of inner electrons that reduces the full nuclear charge. Ions with a higher Z_eff will generally have a smaller radius because the increased attraction between the nucleus and the electrons pulls them closer, resulting in a smaller ionic size.

Ionic size trends can be observed in the periodic table, where cations decrease in size across a period due to increasing nuclear charge, while anions increase in size. Additionally, within a group, ionic size increases down the group due to the addition of electron shells. Understanding these trends is essential for comparing the sizes of different ions, such as Mg2+, O2-, F-, and Na+.