Difference Between Atomic Radius and Ionic Radius

You cant simply whip out a yardstick or ruler to measure the size of an  atom. These  building blocks  of all matter  are much too small, and, since  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. The two are very similar—and in some cases, even the same—but there are minor and important differences between them. Read on to 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 or 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 range from 30 to 300 pm or trillionths of a meter. The atomic radius is a term used to describe the size of the  atom. However, 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 the ionic radius is further complicated by the shape of atoms and ions. While particles of matter are often depicted as spheres, they arent 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 Mendeleevs table but today, elements are ordered by increasing  atomic number, which reflects  the number of protons  in an atom. There arent 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. Fundamentals in Nuclear Physics. Springer. 2005. ISBN 978-0-387-01672-6. Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry (5th ed., p.1385). Wiley. 1988. ISBN 978-0-471-84997-1.Pauling, L. The Nature of the Chemical Bond (3rd ed.). Ithaca, NY: Cornell University Press. 1960Wasastjerna, J. A. On the Radii of Ions.  Comm. Phys.-Math., Soc. Sci. Fenn.  1  (38): 1–25. 1923