Ionic Radius Trends in the Periodic Table

Periodic Table Trends for Ionic Radius

Ionic radius is different from atomic radius of an element.
Ionic radius is different from atomic radius of an element. Science Photo Library - MEHAU KULYK, Getty Images

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 atomic radius as well as to ionic radius.

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 period 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 atoms.