Ionic Radii Calculator

Calculate Ionic Radii with Precision

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Ionic Radius Calculator

Calculate ionic radius based on element, charge state, and coordination number.

Ionic Radius: - pm

Radius Ratio Calculator

Calculate the radius ratio between cation and anion to predict coordination geometry.

Radius Ratio: -
Predicted Geometry: -

Understanding Ionic Radii

What are Ionic Radii?

Ionic radius is the measure of an ion's size in a crystal lattice. It is affected by:

  • Atomic number
  • Charge state
  • Coordination number
  • Electronic configuration
  • Spin state (for transition metals)

Trends in Ionic Radii

Several trends affect ionic radii:

  • Isoelectronic Series: Radius decreases with increasing nuclear charge
  • Period: Radius generally decreases left to right
  • Group: Radius increases down the group
  • Charge: Higher charge means smaller radius

Radius Ratio Rule

The radius ratio (r+/r-) determines coordination geometry:

  • > 0.732: 8-coordinate (cubic)
  • 0.414 - 0.732: 6-coordinate (octahedral)
  • 0.225 - 0.414: 4-coordinate (tetrahedral)
  • < 0.225: 3-coordinate (triangular)

Common Ionic Radii Values

Alkali Metals (CN=6)

Li⁺: 76 pm

Na⁺: 102 pm

K⁺: 138 pm

Alkaline Earth Metals (CN=6)

Mg²⁺: 72 pm

Ca²⁺: 100 pm

Sr²⁺: 118 pm

Common Anions

F⁻: 133 pm

Cl⁻: 181 pm

O²⁻: 140 pm