What Is the Difference Between Atomic Radius and Ionic Radius?

The two are similar, but there are differences

Computer graphic of a Beryllium atom.

You can't simply whip out a meter stick to measure the size of an atom. These building blocks of all matter are much too small. Also, because electrons are always in motion, the diameter of an atom is a bit fuzzy. Two measures used to describe atomic size are atomic radius and ionic radius. They are very similar, and even the same in some cases, but there are minor and important differences between the two. Read on learn more about these two ways to measure an atom.

Atomic Radius

The atomic radius is the distance from the atomic nucleus to the outermost stable electron of a neutral atom. In practice, the value is obtained by measuring the diameter of an atom and dividing it in half. But, it gets trickier from there.

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 actually refer to the ionic radius, as well as the covalent radius, metallic radius, or van der Waals radius.

Ionic Radius

The ionic radius is half the distance between two gas atoms that are just touching each other. In a neutral atom, the atomic and ionic radius are the same, but many elements exist as anions or cations. If the atom loses its outermost electron (positively charged or cation), the ionic radius is smaller than the atomic radius because the atom loses an electron energy shell. If the atom gains an electron (negatively charged or anion), usually the electron falls into an existing energy shell so the size of the ionic radius and atomic radius are comparable.

Trends in the Periodic Table

Whichever method you use to describe atomic size, it displays a trend or periodicity in the periodic table. Periodicity refers to the recurring trends that are seen in the element properties. These trends became apparent to Demitri Mendeleev when he arranged the elements in order of increasing mass. Based on the properties that were displayed by the known elements, Mendeleev was able to predict where there were holes in his table, or elements yet to be discovered.

The modern periodic table is very similar to Mendeleev's table, but today elements are ordered by increasing atomic number, which reflects the number of protons in an atom. There aren't any undiscovered elements, although new elements can be created that have even higher numbers of protons.

Atomic and ionic radius increase as you move down a column (group) of the periodic table because an electron shell is added to the atoms. Atomic size decreases as you move across a row—or period—of the table because the increased number of protons exerts a stronger pull on the electrons. Noble gasses are the exception. Although the size of a noble gas atom does increase as you move down the column, these atoms are larger than the preceding atoms in a row.