Tutorials

Ionic Radius Trends

Core Concepts

In this tutorial, you will be introduced to ionic radius trends on the periodic table of elements. You will also be introduced to the concepts that contribute to ionic radius, including how to find it.

Topics Covered in Other Articles

Vocabulary

  • Atomic Radius – The atomic radius of a chemical element is a measure of the size of its atom, usually the mean or typical distance from the center of the nucleus to the outermost isolated electron. 
  • Ionic Radius – is the radius of a monatomic ion in an ionic crystal structure. Although neither atoms nor ions have sharp boundaries, they are treated as if they were hard spheres with radii such that the sum of ionic radii of the cation and anion gives the distance between the ions in a crystal lattice.
  • Valence Electrons– the electrons in the outermost shell, or energy level, of an atom.

What is an Ion?

An ion is an atom that carries a positive or negative electric charge. When an atom on the periodic table loses an electron it becomes a cation. In contrast, when an atom in the periodic table gains an electron it becomes an anion.

Most metals on the periodic table tend to form cations. For example, the metals in group 1A (Alkali Metals) all have a +1 charge meaning they want to give away an electron. The non-metals tend to become anions.

Group 1A elements

Ionic Radius

By definition, an ionic radius is the distance of the outermost shell of electrons (valence shell) from the nucleus of an ion. So in the picture below the ionic radius would be found by measuring the distance between the yellow and blue protons in the nucleus. To the red electrons at the outermost orbital. This is a good visualization of how the radius may be measured but this is unlikely to be accurate because the borders of orbitals are quite fuzzy.

Valence Electrons
“How to Find the Number of Valence Electrons?” Science ABC, 17 Jan. 2022, https://www.scienceabc.com/pure-sciences/how-to-find-the-number-of-valence-electrons-in-an-element.html.

The ionic radius of an ion is measured when the atom is in a crystal lattice structure. The ionic radius is half of the distance between two ions that is just touching one another. As mentioned above, the shell boundaries of electron shells are difficult to read. So the ions of an atom are treated as if they were spheres. The ionic radius can easily be a little smaller or larger than the atomic radius, which is the radius a neutral atom of the element possesses.

Finding Ionic Radius

Since ions tend to exist in bonds, the ionic radius can be found via the ionic bond between two atoms. Moreover, the ionic radius is tricky to measure since it depends on the varying factors of the environment in which the ion is located. It depends on the number of atoms, ions, or molecules that a central atom or ion holds as its nearest neighbors in a complex or coordination compound. It also depends on the spin state of the ion. The ionic radius is generally calculated by estimating the distance between the two nuclei and dividing it according to the atomic sizes. Ionic radius is generally measured in picometer (pm) or nanometer (nm).

Ionic Radius Example

Cations Vs. Anions

Because cations lose electrons from the valence shell they often have smaller ionic radii than their parent neutral atom. Vice Versa for anions since anions gain electrons to the outermost shell they usually will have bigger radii than their parent neutral atom.

Ex. The atomic radius of sodium (Na) is 190 pm, but the ionic radius of sodium (Na+1) is only 116 pm.

Ex. The atomic radius of the chlorine atom (Cl) is 79 pm, and the ionic radius of the chlorine ion (Cl-1) is 167 pm.

Ionic Radius Trends in the Periodic Table

Ionic radius and Group

As you move down the periodic table additional electrons are being added, which in turn causes the ionic radius to increase.

Ionic radius trends

Look at group 2 on the periodic table (Alkaline Earth Metals)

  • Beryllium ionic radius – 31 pm
  • Magnesium ionic radius – 65 pm
  • Calcium ionic radius – 99 pm
  • Strontium ionic radius – 113 pm
  • Barium ionic radius – 135 pm

Congruent with the trend of increasing atomic radius as you move down the periodic table.

Ionic Radius and Period

As you move across the periodic table more electrons, protons, and neutrons are being added. So it would make sense for the radius to increase, but it does not. This is because as you move over a row on the periodic table the ionic radius decreases for metals forming cations. The ionic radius increases for nonmetals as the effective nuclear charge decreases.

Moving across the periodic table from potassium (K)

  • Potassium (K) ionic radius – 137 pm
  • Calcium ionic (Ca) radius – 99 pm
  • Scandium ionic (Sc) radius – 87 pm

Notice as you move to the right on the periodic table the ionic radius size is decreasing.