Ionic Radius Trends in the Periodic Table

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The ionic radius of the elements exhibits trends in the periodic table. In general:

  • Ionic radius increases as you move from top to bottom on the periodic table.
  • Ionic radius decreases as you move across the periodic table, from left to right.

Although ionic radius and atomic radius do not mean exactly the same thing, the trend applies to the atomic radius as well as to the ionic radius.

Key Takeaways: Ionic Radius Trend on Periodic Table

  • The ionic radius is half the distance between atomic ions in a crystal lattice. To find the value, ions are treated as if they were hard spheres.
  • The size of an element's ionic radius follows a predictable trend on the periodic table.
  • As you move down a column or group, the ionic radius increases. This is because each row adds a new electron shell.
  • Ionic radius decreases moving from left to right across a row or period. More protons are added, but the outer valence shell remains the same, so the positively charged nucleus draws in the electrons more tightly. But, for the nonmetallic elements, the ionic radius increases because there are more electrons than protons.
  • While the atomic radius follows a similar trend, ions may be larger or smaller than neutral atoms.

Ionic Radius and Group

Why does radius increase with higher atomic numbers in a group? As you move down a group in the periodic table, additional layers of electrons are being added, which naturally causes the ionic radius to increase as you move down the periodic table.

Ionic Radius and Period

It might seem counterintuitive that the size of an ion would decrease as you add more protons, neutrons, and electrons in a period, yet, there's an explanation for this. As you move across a row of the periodic table, the ionic radius decreases for metals forming cations, as the metals lose their outer electron orbitals. The ionic radius increases for nonmetals as the effective nuclear charge decreases due to the number of electrons exceeding the number of protons.

Ionic Radius and Atomic Radius

The ionic radius is different from the atomic radius of an element. Positive ions are smaller than their uncharged atoms. Negative ions are larger than their neutral atoms.

Sources

  • Pauling, L. The Nature of the Chemical Bond. 3rd ed. Cornell University Press, 1960.
  • Wasastjerna, J. A. "On the radii of ions." Comm. Phys.-Math., Soc. Sci. Fenn. vol. 1, no. 38, pp. 1–25, 1923.