Ionic radii

Oct 31, 2023

The concept of ionic radii is a fundamental aspect of chemistry that plays a crucial role in understanding the behavior of ionic compounds. Ionic radii provide insights into the size and charge of ions, which are the building blocks of ionic compounds. In this article, we will delve into the intriguing world of ionic radii, exploring their significance, factors influencing them, and the implications for various chemical phenomena.

Understanding Ionic Radii

Ionic radii are defined as the effective sizes of ions in ionic compounds. These radii are primarily used to compare the relative sizes of ions and to predict their arrangements in crystal lattices. The size of an ion is determined by its electronic configuration, the number of electrons it has gained or lost, and its charge. When atoms lose or gain electrons to form ions, their size and charge change, resulting in ionic radii that differ from the atomic radii of neutral atoms.

Factors Affecting Ionic Radii

Several factors influence the ionic radii of ions. Some of the key determinants include:

  1. Number of Electrons: As ions gain or lose electrons, the number of electrons in the outermost electron shell changes, affecting the electron-electron repulsion and hence the ionic radius.
  2. Charge of the Ion: The magnitude of the charge on an ion plays a significant role in determining its ionic radius. Cations (positively charged ions) are smaller than their parent atoms, while anions (negatively charged ions) are larger.
  3. Position on the Periodic Table: The location of an element on the periodic table can influence the size of its ions. Elements in the same group tend to form ions with similar sizes, while those in different periods may have more significant differences.
  4. Electron Configuration: The distribution of electrons in the electron shells of an ion affects its ionic radius. Ions with completely filled or half-filled electron shells tend to be more stable and may have different radii compared to ions with partially filled shells.

Ionic Radii Data

Let’s explore some examples of common ions and their corresponding ionic radii. Here are a few tables to illustrate these values:

Table 1: Ionic Radii of Selected Cations (in picometers, pm)

IonIonic Radius (pm)
Na+ (Sodium)95
K+ (Potassium)133
Mg2+ (Magnesium)65
Ca2+ (Calcium)100
Al3+ (Aluminum)54

Table 2: Ionic Radii of Selected Anions (in picometers, pm)

IonIonic Radius (pm)
F- (Fluoride)133
O2- (Oxide)140
Cl- (Chloride)181
S2- (Sulfide)184
N3- (Nitride)146

These tables showcase the differences in ionic radii for various cations and anions, highlighting how the charge and atomic structure affect ion size.

Significance in Chemistry

Ionic radii are essential in various chemical phenomena, including the determination of crystal structures, the prediction of solubility, and the understanding of ion exchange in zeolites and ion-selective membranes. These values also aid in explaining trends in properties such as melting points, boiling points, and ionic conductivities within ionic compounds.


The concept of ionic radii provides valuable insights into the behavior of ions and their interactions in the world of chemistry. By understanding the factors that influence ionic radii and studying the sizes of various ions, scientists and researchers can make predictions about a wide range of chemical phenomena, aiding the development of new materials and the advancement of chemistry as a whole.

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