Atomic Radius Definition and Trend

Chemistry Glossary Definition of Atomic Radius

The atomic radius is used to describe the size of an atom.
The atomic radius is used to describe the size of an atom. Getty Images

Atomic Radius Definition

The atomic radius is a term used to describe the size of the atom, but there is no standard definition for this value. Atomic radius may refer to the ionic radius, covalent radius, metallic radius, or van der Waals radius.

Atomic Radius Periodic Table Trend

No matter what criteria you use to describe the atomic radius, the size of the atom is dependent on how far out the electrons extend.

The atomic radius for an element tends to increase as one goes down an element group. The reason is that the electrons become more tightly packed as you move across the periodic table, so while there are more electrons for elements of increasing atomic number, the atomic radius actually may decrease. The atomic radius moving down an element period or column tends to increase because an additional electron shell is added for each new row. In general, the largest atoms are at the bottom left side of the periodic table.

Atomic Radius Versus Ionic Radius

The atomic and ionic radius is the same for atoms of elements that are neutral, such as argon, krypton, and neon. However, many atoms of elements are more stable as atomic ions. If the atom loses its outermost electron, it becomes a cation or positively charged ion. Examples include K+ and Na+. Some atoms may even lose multiple outer electrons, such as Ca2+.

When electrons are removed from an atom, it may lose its outermost electron shell, making the ionic radius smaller than the atomic radius. In contrast, some atoms are more stable if they gain one or more electrons, forming an anion or negatively charged atomic ion. Examples include Cl- and F-. Because another electron shell isn't added, the size difference between the atomic radius and ionic radius of an anion isn't as much as for a cation.

The anion ionic radius is the same or slightly larger than the atomic radius.

Overall, the trend for the ionic radius is the same as for the atomic radius (increasing in size moving across and decreasing moving down the periodic table). However, it's important to keep in mind it's tricky to measure the ionic radius, not least because charged atomic ions repel each other!

How Atomic Radius Is Measured

Let's face it. You can't just put atoms under a normal microscope and measure their size (although this kind of works using an atomic force microscope). Also, atoms don't sit still for examination. They are constantly in motion. Thus, any measure of atomic (or ionic) radius is an estimate that contains a large amount of error. The atomic radius is measured based on the distance between the nuclei of two atoms that are just barely touching each other. In other words, this means the electron shells of the two atoms are just touching each other. This diameter between the atoms is divided by two to give the radius.

It is important the two atoms don't share a chemical bond (e.g., O2, H2) because the bond implies an overlap of the electron shells or a shared outer shell.

The atomic radii of atoms cited in the literature is usually empirical data taken from crystals.

For newer elements, the atomic radii are theoretical or calculated values, based on the probable size of the electron shells. In case you're wondering how big an atom is, the atomic radius of the hydrogen atom is about 53 picometers. The atomic radius of an iron atom is about 156 picometers. The largest (measured) atom is cesium, which has a radius of about 298 picometers.

Reference

Slater, J. C. (1964). "Atomic Radii in Crystals". Journal of Chemical Physics. 41 (10): 3199–3205.