What Is the Difference Between Atomic Radius and Ionic Radius?

The two are similar, but there are differences

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. 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.

Key Takeaways: Atomic vs Ionic Radius

  • There are different ways to measure the size of the atom, including atomic radius, ionic radius, covalent radius, and van der Waals radius.
  • The atomic radius is half the diameter of a neutral atom. In other words, it is half the diameter of an atom, measuring across the outer stable electrons.
  • The ionic radius is half the distance between two gas atoms that are just touching each other. This value may be the same as the atomic radius, or it may be larger for anions and the same size of smaller for cations.
  • Both atomic and ionic radius follow the same trend on the periodic table. Generally, radius decreases moving across a period (row) and increases moving down a group (column).

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. The radii of neutral atoms ranges from 30 to 300 pm or trillionths of a meter.

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. Values range from 30 pm to over 200 pm. 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.

The concept of ionic radius is further complicated by the shape of atoms and ions. While particles of matter are often depicted as spheres, they aren't always round. Researchers have discovered chalcogen ions are actually ellipsoid in shape.

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.

Sources

  • Basdevant, J.-L.; Rich, J.; Spiro, M. (2005). Fundamentals in Nuclear Physics. Springer. ISBN 978-0-387-01672-6.
  • Cotton, F. A.; Wilkinson, G. (1988). Advanced Inorganic Chemistry (5th ed.). Wiley. p. 1385. ISBN 978-0-471-84997-1.
  • Pauling, L. (1960). The Nature of the Chemical Bond (3rd Edn.). Ithaca, NY: Cornell University Press.
  • Wasastjerna, J. A. (1923). "On the radii of ions". Comm. Phys.-Math., Soc. Sci. Fenn1 (38): 1–25.