Ionic Radius
Definition and meaning of Ionic Radius in chemistry.
Ionic radius is the effective radius of an ion in a crystal lattice, measured as the distance from the ion's nucleus to the point where its electron cloud meets that of a neighboring, oppositely charged ion.
In more detail
Because ions cannot be isolated and measured individually, ionic radii are derived from X-ray diffraction data on interionic distances in ionic solids, then apportioned between the cation and anion using a reference scale such as Shannon's effective ionic radii. Cations are always smaller than their parent atoms because losing electrons increases the effective nuclear charge felt by the remaining electrons and often eliminates an entire occupied shell. Anions are always larger than their parent atoms because added electrons increase electron-electron repulsion, causing the electron cloud to expand. Within an isoelectronic series (same electron count), radius decreases as nuclear charge increases.
Key facts
| Field | General Chemistry |
|---|---|
| Typical unit | picometers (pm) |
| Cation vs. anion size | cation < parent atom < anion |
| Group trend (same charge) | increases down a group |
The sodium atom has a radius of about 186 pm, but the Na+ ion, formed by losing its single 3s electron and exposing the smaller, tightly held neon core, shrinks to about 102 pm. In contrast, the chlorine atom (99 pm) expands to the Cl- ion (181 pm) after gaining an electron.
Frequently asked questions
Why is a cation smaller than its neutral atom?
Removing one or more electrons increases the ratio of protons to remaining electrons (higher effective nuclear charge per electron), and often removes the outermost occupied shell entirely, so the remaining electron cloud is pulled in closer to the nucleus.
How do chemists determine ionic radii if ions cannot be isolated?
They measure the total interionic distance between adjacent cations and anions in a crystal by X-ray diffraction, then divide that distance between the two ions using a self-consistent reference scale, such as the Shannon effective ionic radii tables.