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.

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