Ionic Radius Definition and Trend

The ionic radius (plural: ionic radii) is the measure of an atom's ion in a crystal lattice. It is half the distance between two ions that are barely touching each other. Since the boundary of the electron shell of an atom is somewhat fuzzy, the ions are often treated as though they were solid spheres fixed in a lattice.

The ionic radius may be larger or smaller than the atomic radius (radius of a neutral atom of an element), depending on the electric charge of the ion. Cations are typically smaller than neutral atoms because an electron is removed and the remaining electrons are more tightly drawn in toward the nucleus. An anion has an additional electron, which increases the size of the electron cloud and may make the ionic radius larger than the atomic radius.

Values for ionic radius are difficult to obtain and tend to depend on the method used to measure the size of the ion. A typical value for an ionic radius would be from 30 picometers (pm, and equivalent to 0.3 Angstroms Å) to 200 pm (2 Å). Ionic radius may be measured ​using x-ray crystallography or similar techniques.

Ionic Radius Trend in the Periodic Table

Ionic radius and atomic radius follow the same trends in the periodic table:

• As you move from top to bottom down an element group (column) ionic radius increases. This is because a new electron shell is added as you move down the periodic table. This increases the overall size of the atom.
• As you move from left to right across an element period (row) the ionic radius decreases. Even though the size of the atomic nucleus increases with larger atomic numbers moving across a period, the ionic and atomic radius decreases. This is because the effective positive force of the nucleus also increases, drawing in the electrons more tightly. The trend is particularly obvious with the metals, which form cations. These atoms lose their outermost electron, sometimes resulting in the loss of an entire electron shell. The ionic radius of transition metals in a period does not, however, change very much from one atom to the next near the beginning of a series.

Variations in Ionic Radius

Neither the atomic radius nor the ionic radius of an atom is a fixed value. The configuration or stacking of atoms and ions affects the distance between their nuclei. The electron shells of atoms can overlap each other and do so by different distances, depending on the circumstances.

The "just barely touching" atomic radius is sometimes called the van der Waals radius since the weak attraction from van der Waals forces governs the distance between the atoms. This is the type of radius commonly reported for noble gas atoms. When metals are covalently bonded to each other in a lattice, the atomic radius may be called the covalent radius or the metallic radius. The distance between nonmetallic elements may also be termed the covalent radius.

When you read a chart of ionic radius or atomic radius values, you're most likely seeing a mixture of metallic radii, covalent radii, and van der Waals radii. For the most part, the tiny differences in the measured values shouldn't be a concern. What's important is understanding the difference between atomic and ionic radius, the trends in the periodic table, and the reason for the trends.