Ionic Radii

Ionic Radii

• Definition: Ionic radii refer to the size of an ion, which is influenced by the addition or removal of electrons from an atom.

Formation of Ions

• Cations: Formed when electrons are removed from an atom.

  • Charge: Resulting ion is positive.

  • Removed Electron: Typically the outermost valence electron.

• Anions: Formed when electrons are added to an atom.

  • Charge: Resulting ion is negative.

  • Added Electrons: Fill the next available orbital, lowest in energy.

Comparison of Atomic and Ionic Radii

• Practice Question: Which has a larger radius, the lithium atom or the lithium cation?

  • Answer: The lithium atom is larger.

  • Explanation:

    • In the lithium atom, there are 3 protons pulling on 3 electrons.

    • In the lithium cation, there are 3 protons pulling on only 2 electrons.

    • Less electron-electron repulsion in the cation results in a tighter pull and a smaller radius.

Cation Size Trend

• Cations are always smaller than their corresponding neutral atoms.

Anions and Their Size

• Practice Question: Which has a larger radius, the fluorine atom or the fluoride anion?

  • Answer: The fluoride anion is larger.

  • Why?: Adding an electron increases electron-electron repulsion, causing the electrons to push apart, resulting in a larger radius for the anion compared to the neutral atom.

Anion Size Trend

• Anions are larger than their parent neutral atoms.

Isoelectronic Series

• Definition: An isoelectronic series consists of atoms and ions that have the same electron configuration but different numbers of protons.

  • Example: Sodium cation vs. Fluoride anion.

  • Comparison: Fluoride anion is larger despite both having the same electron configuration as Neon because the fluoride anion has fewer protons (9) compared to sodium (11).

  • Trend: The more protons there are, the stronger the attractive force on the electrons, pulling them closer to the nucleus and leading to a smaller size of the ion.

Conclusion

• Understanding the size difference between cations and anions is crucial.

• Remember that trends in ionic radius are based on the interplay of electron-electron repulsion and nucleus-electron attraction.