The correct order of increasing size for the ions F^-, Na⁺, O^2-, Al³⁺, Mg²⁺ is

CONCEPT:

Trends in Ionic Sizes

• Ionic size is influenced by the number of protons, electrons, and electron-electron repulsion in an ion.
• In general:
  • Cations are smaller than their parent atoms due to the loss of electrons, resulting in reduced electron-electron repulsion and stronger attraction of the remaining electrons to the nucleus.
  • Anions are larger than their parent atoms due to the gain of electrons, increasing electron-electron repulsion and slightly weakening the attraction of the electrons to the nucleus.
• For isoelectronic species (ions with the same number of electrons), the ionic size decreases as the nuclear charge increases. This is because a higher nuclear charge pulls the electrons closer to the nucleus.

EXPLANATION:

• The ions F^-, Na⁺, O^2-, Al³⁺, and Mg²⁺ are isoelectronic (all have 10 electrons).
• The nuclear charges of these ions are as follows:
  • Al³⁺: +13
  • Mg²⁺: +12
  • Na⁺: +11
  • F^-: +9
  • O^2-: +8
• Higher nuclear charge results in stronger attraction of electrons to the nucleus, leading to smaller ionic size.
• Therefore, the correct order of increasing ionic size is:

  Al³⁺ < Mg²⁺ < Na⁺ < F^- < O^2-

Therefore, the correct answer is Al³⁺ < Mg²⁺ < Na⁺ < F^- < O^2-.