Ranking Ionic Radii and Isoelectronic Species in GOB Chemistry

Ranking Ionic Radii and Isoelectronic Species

Introduction

Understanding how to compare the sizes of ions and atoms is essential in General, Organic, and Biological (GOB) Chemistry. This topic focuses on ranking the ionic radii of various ions, especially those that are isoelectronic—ions or atoms with the same number of electrons.

Key Concepts

• Ionic Radius: The radius of an ion, which can vary depending on the ion's charge and the number of electrons.

• Isoelectronic Species: Atoms or ions that have the same number of electrons, but may differ in nuclear charge (number of protons).

Steps for Ranking Ionic Radii

1. Determine the Total Number of Electrons: For each atom or ion, calculate the total number of electrons.

2. Compare Charges for Isoelectronic Species: If the species are isoelectronic, the ion with the most negative charge will have the largest radius, while the ion with the most positive charge will have the smallest radius.

Example: Ranking Isoelectronic Ions

Example: Arrange the atoms and/or ions in order of decreasing ionic radius: Fe3+, Mn2+, Ni2+, Zn2+.

• All these ions are isoelectronic (same number of electrons).

• The more positive the charge, the smaller the ionic radius due to increased attraction between electrons and the nucleus.

Isoelectronic Species: Ranking by Ionic Radius

For isoelectronic species, the order of ionic radius is determined by the charge:

• Most negative charge → Largest radius

• Most positive charge → Smallest radius

General trend for isoelectronic ions (example):

$\text{O}^{2-} > \text{F}^- > \text{Na}^+ > \text{Mg}^{2+} > \text{Al}^{3+}$

Practice Problems

• Arrange the following atoms and/or ions in order of increasing size: Br-, Kr, Rb+, Se2-

• Arrange the following isoelectronic series in order of decreasing radius: F-, O2-, Mg2+, Na+

Practice: Smallest Ion in an Isoelectronic Series

For an isoelectronic series of ions, the ion that is the smallest is always:

• (a) The ion with the fewest protons. ✓ Correct

• (b) The least positively charged ion.

• (c) The ion with the highest atomic number.

• (d) The ion with the most neutrons.

Explanation: In an isoelectronic series, all species have the same number of electrons. The ion with the most protons (highest positive charge) pulls the electrons closer, resulting in the smallest radius.

Summary Table: Isoelectronic Series and Ionic Radii

Ion	Charge	Number of Electrons	Relative Size
O2-	-2	10	Largest
F-	-1	10	Smaller
Na+	+1	10	Even smaller
Mg2+	+2	10	Smallest

Key Takeaways

• When ranking ionic radii, always consider the number of electrons and the nuclear charge.

• For isoelectronic species, the more positive the charge, the smaller the radius.

• Practice identifying isoelectronic series and applying these trends to rank ionic sizes.