How does ionic size vary for cations and anions?

Short Answer

Ionic size varies depending on whether an ion is a cation or an anion.

  • Cations are smaller than their parent atoms because they lose electrons, reducing electron-electron repulsion and allowing the nucleus to pull electrons closer.
  • Anions are larger than their parent atoms because they gain electrons, increasing repulsion among electrons and expanding the electron cloud.
Detailed Explanation :

Cations and Their Size

  1. Formation: Cations are formed when an atom loses one or more electrons, usually from the outermost shell.
  2. Effect on Radius:
    • Losing electrons reduces the number of electron shells or decreases electron-electron repulsion.
    • The effective nuclear charge (protons pulling on fewer electrons) increases, pulling electrons closer to the nucleus.
    • Result: smaller ionic radius than the neutral atom.
  3. Examples:
    • Sodium atom (Na) → 186 pm, Sodium ion (Na⁺) → 102 pm
    • Magnesium atom (Mg) → 160 pm, Magnesium ion (Mg²⁺) → 72 pm
  4. Trend Across a Period: Cation size decreases from left to right because nuclear charge increases while electrons are in the same shell.
  5. Trend Down a Group: Cation size increases down a group because additional electron shells are added.

Anions and Their Size

  1. Formation: Anions are formed when an atom gains one or more electrons, usually to complete its valence shell.
  2. Effect on Radius:
    • Gaining electrons increases electron-electron repulsion in the outer shell.
    • The number of protons remains the same → nuclear pull per electron decreases.
    • Result: larger ionic radius than the neutral atom.
  3. Examples:
    • Chlorine atom (Cl) → 99 pm, Chloride ion (Cl⁻) → 181 pm
    • Oxygen atom (O) → 60 pm, Oxide ion (O²⁻) → 140 pm
  4. Trend Across a Period: Anion size decreases across a period due to increasing nuclear charge.
  5. Trend Down a Group: Anion size increases down a group because new electron shells are added.

Comparison Between Cations and Anions

  • Cations are always smaller than their parent atoms.
  • Anions are always larger than their parent atoms.
  • Within a period, cations are on the left, anions on the right of the periodic table.
  • Higher charge cations (like Al³⁺) are much smaller than lower charge cations (like Na⁺).
  • Higher charge anions (like O²⁻) are larger than lower charge anions (like F⁻).

Factors Affecting Ionic Size

  1. Charge of Ion: More positive → smaller radius; more negative → larger radius.
  2. Number of Electron Shells: More shells → larger radius.
  3. Nuclear Charge: Stronger nuclear pull → smaller radius.
  4. Electron-Electron Repulsion: Greater repulsion → larger radius.

Significance of Ionic Size Variation

  1. Predicting Bond Lengths: Determines distances in ionic compounds.
  2. Explaining Reactivity: Smaller cations form stronger bonds with anions; larger anions influence solubility.
  3. Crystal Lattice Stability: Ionic size affects packing and lattice energy.
  4. Trends in the Periodic Table: Helps understand periodic trends in chemical properties.
  5. Industrial Applications: Useful for material design, chemical synthesis, and electronics.

Examples of Trends

  • Period 3 Elements: Na⁺ (102 pm) < Mg²⁺ (72 pm) < Al³⁺ (53 pm) < Cl⁻ (181 pm) < S²⁻ (184 pm)
  • Group 17 Elements: F⁻ < Cl⁻ < Br⁻ < I⁻ → size increases down the group.
Conclusion

Ionic size varies significantly between cations and anions. Cations are smaller than their parent atoms due to electron loss and stronger nuclear pull, while anions are larger due to electron gain and increased repulsion. Understanding this variation is crucial for explaining bond lengths, lattice structures, reactivity, and periodic trends, making it a key concept in chemistry and material science.