What is ionic radius?

Short Answer

Ionic radius is the distance from the nucleus of an ion to its outermost electron. It determines the size of an ion, which can be different from the size of its parent atom.

  • Cations (positively charged ions) are smaller than the neutral atom because they lose electrons, reducing electron-electron repulsion.
  • Anions (negatively charged ions) are larger than the neutral atom because they gain electrons, increasing repulsion among electrons.
Detailed Explanation :

Definition of Ionic Radius

Ionic radius is defined as the distance from the nucleus of an ion to the outermost electron shell. It gives a measure of the size of the ion, which is crucial for understanding bond lengths, lattice structures, and chemical reactivity.

  • Unit: picometers (pm) or angstroms (Å)
  • Depends on the charge of the ion and the number of electron shells.

Difference Between Atomic Radius and Ionic Radius

  1. Atomic Radius: Distance from nucleus to the outermost electron in a neutral atom.
  2. Ionic Radius: Distance from nucleus to the outermost electron in an ion (either cation or anion).
  3. Trend: Cations are smaller, anions are larger than their parent atoms.

Cations and Ionic Radius

  • Formed by losing electrons.
  • Number of protons > number of electrons → stronger nuclear pull on remaining electrons → smaller radius.
  • Example:
    • Sodium (Na) → 186 pm
    • Sodium ion (Na⁺) → 102 pm
  • The loss of an outer shell or reduction in electron-electron repulsion reduces the size.

Anions and Ionic Radius

  • Formed by gaining electrons.
  • Number of electrons > number of protons → weaker nuclear pull → electrons spread out → larger radius.
  • Example:
    • Chlorine (Cl) → 99 pm
    • Chloride ion (Cl⁻) → 181 pm
  • Increase in electron repulsion expands the electron cloud.

Trend in Ionic Radius Across a Period

  1. Cations: Size decreases across a period due to increasing nuclear charge while the number of electrons is constant.
  2. Anions: Size also decreases across a period, as nuclear charge increases and pulls electrons closer.
  3. Cations appear on the left, anions on the right of the periodic table.

Trend in Ionic Radius Down a Group

  1. Ionic radius increases down a group for both cations and anions.
  2. Reason: Additional electron shells are added as atomic number increases → outer electrons farther from nucleus.

Factors Affecting Ionic Radius

  1. Charge of the Ion: Higher positive charge → smaller radius; higher negative charge → larger radius.
  2. Number of Electron Shells: More shells → larger radius.
  3. Nuclear Charge: More protons pull electrons closer, reducing radius.
  4. Electron-Electron Repulsion: More electrons → increased repulsion → larger radius.

Significance of Ionic Radius

  1. Predicting Bond Lengths: Determines distance between ions in ionic compounds.
  2. Explaining Reactivity: Smaller cations have stronger attraction to anions, affecting reactivity.
  3. Crystal Lattice Structures: Ionic radii influence the stability and geometry of ionic crystals.
  4. Periodic Trends: Helps explain trends in ionization energy, electronegativity, and metallic character.
  5. Chemical Applications: Essential in chemistry, material science, and industrial processes.

Examples

  1. Sodium Ion (Na⁺): Smaller than Na atom, forms stable ionic crystals with Cl⁻.
  2. Chloride Ion (Cl⁻): Larger than Cl atom, combines with Na⁺ to form NaCl.
  3. Magnesium Ion (Mg²⁺): Smaller than Mg atom due to loss of two electrons.
Conclusion

Ionic radius measures the size of an ion, reflecting how electron gain or loss affects atomic size. Cations are smaller than their parent atoms, while anions are larger. Ionic radius is crucial for predicting bond lengths, lattice structures, reactivity, and periodic trends, making it an important concept in chemistry, material science, and industrial applications.