What Is the Difference Between Atomic Radius and Ionic Radius?

Atomic vs Ionic Radius

Computer graphic of a Beryllium atom.
MEHAU KULYK/SCIENCE PHOTO LIBRARY/Getty Images

You can't simply whip out a meter stick to measure the size of an atom! Also, because electrons are always in motion, the diameter of an atom is a bit fuzzy. Two measures used to describe atomic size areĀ atomic radius and ionic radius. Here is an explanation of the difference between them.

Atomic Radius (plural Atomic Radii)

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.

Ionic Radius (plural Ionic Radii)

The ionic radius is one-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.

Trend in the Periodic Table

Whichever method you use to describe atomic size, it displays a trend or periodicity in the periodic table. 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 (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.

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Helmenstine, Anne Marie, Ph.D. "What Is the Difference Between Atomic Radius and Ionic Radius?" ThoughtCo, Feb. 6, 2017, thoughtco.com/difference-between-atomic-radius-and-ionic-radius-603819. Helmenstine, Anne Marie, Ph.D. (2017, February 6). What Is the Difference Between Atomic Radius and Ionic Radius? Retrieved from https://www.thoughtco.com/difference-between-atomic-radius-and-ionic-radius-603819 Helmenstine, Anne Marie, Ph.D. "What Is the Difference Between Atomic Radius and Ionic Radius?" ThoughtCo. https://www.thoughtco.com/difference-between-atomic-radius-and-ionic-radius-603819 (accessed November 22, 2017).