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Atomic Radius vs. Ionic Radius: What's the Difference?

Edited by Aimie Carlson || By Harlon Moss || Published on January 7, 2024
Atomic radius refers to the distance from the nucleus to the outermost electron shell in a neutral atom, whereas ionic radius is the radius of an atom's ion, altered by loss or gain of electrons.

Key Differences

Atomic radius and ionic radius are key concepts in chemistry, each defining the size of an atom or ion. The atomic radius is the distance from an atom's nucleus to its outermost electron shell in a neutral state. In contrast, the ionic radius is the size of an atom's ion, which changes when the atom gains or loses electrons, becoming either negatively or positively charged.
The atomic radius is determined by the number of electron shells and the effective nuclear charge. As the atomic number increases, the atomic radius usually increases due to the addition of more electron shells. However, the ionic radius is influenced by the atom's charge; cations (positively charged ions) typically have a smaller radius than the original atom due to the loss of an electron shell, while anions (negatively charged ions) have a larger radius due to increased electron-electron repulsion.
Trends in atomic radius are observed across the periodic table. Elements in the same group (vertical column) generally have increasing atomic radii down the group, while elements in the same period (horizontal row) have decreasing atomic radii across the period. The ionic radius, however, varies based on the specific ionization process and is not as straightforward, often deviating from the trends seen in atomic radii.
The measurement of atomic radius is usually done through X-ray crystallography or other spectroscopic methods in a neutral atom. In contrast, the ionic radius is often measured in crystalline ionic compounds, where ions are in a lattice structure. This difference in measurement contexts reflects the distinct nature of atomic and ionic radii.
Both atomic and ionic radii play significant roles in understanding chemical bonding and reactivity. The atomic radius is crucial for predicting covalent bonding distances and van der Waals forces, while the ionic radius is important in determining the properties of ionic compounds, including their solubility, crystal structure, and melting points.

Comparison Chart


Distance from nucleus to outermost electron shell in a neutral atom
Radius of an atom's ion, altered by electron loss or gain

Influencing Factors

Number of electron shells, effective nuclear charge
Ion charge, electron-electron repulsion in ions

Trends in Periodic Table

Increases down a group, decreases across a period
Varies based on ionization, not straightforward

Measurement Method

X-ray crystallography, spectroscopy in neutral atoms
Measured in crystalline ionic compounds

Role in Chemistry

Predicts covalent bonding distances, van der Waals forces
Determines properties of ionic compounds

Atomic Radius and Ionic Radius Definitions

Atomic Radius

Atomic radius decreases across a period.
Oxygen has a smaller atomic radius compared to beryllium.

Ionic Radius

Ionic radius is the size of an ion in a crystal lattice.
The ionic radius of Na+ is smaller than that of Na.

Atomic Radius

It's influenced by the number of electron shells.
The atomic radius of elements increases with higher shell numbers.

Ionic Radius

Cations have smaller ionic radii than their neutral atoms.
The ionic radius of Mg2+ is smaller than a magnesium atom.

Atomic Radius

The atomic radius determines van der Waals forces.
Larger atomic radii result in weaker van der Waals forces.

Ionic Radius

The ionic radius impacts solubility and melting points.
Smaller ionic radii generally lead to higher melting points in salts.

Atomic Radius

Atomic radius is the distance from an atom's nucleus to its outermost electron.
The atomic radius of helium is smaller than that of neon.

Ionic Radius

Anions have larger ionic radii than their neutral atoms.
Chloride ions have a larger ionic radius than chlorine atoms.

Atomic Radius

It increases down a group in the periodic table.
Potassium has a larger atomic radius than lithium.

Ionic Radius

Ionic radius affects the crystal structure of salts.
The ionic radius ratio influences the formation of cubic or tetrahedral lattices.


How does atomic radius change across the periodic table?

Decreases across a period, increases down a group.

What is ionic radius?

The size of an ion in a crystal lattice.

What is atomic radius?

Distance from an atom's nucleus to its outermost electron shell.

How do you measure ionic radius?

In ionic compounds within a crystal lattice.

Why do cations have a smaller ionic radius?

Loss of an electron shell reduces size.

Why are anions larger in ionic radius?

Added electrons increase electron-electron repulsion.

What affects the ionic radius?

The charge of the ion and electron-electron repulsion.

How is atomic radius measured?

Through methods like X-ray crystallography.

What role does atomic radius play in chemistry?

Important for understanding bonding and molecular structure.

Do ionic radii follow clear trends like atomic radii?

Trends are less straightforward due to varying ion charges.

What's the difference in atomic radius among isotopes?

Generally negligible, as isotopes have the same electron configuration.

How does ionic radius influence crystal structure?

Determines the type of crystal lattice formed.

What's a notable trend for atomic radius in the periodic table?

Smaller radii for elements with higher atomic numbers in a period.

How does the effective nuclear charge affect atomic radius?

Higher charge pulls electrons closer, reducing radius.

Can atomic radius predict chemical reactivity?

Yes, it helps in predicting reactivity trends.

Why is atomic radius important in van der Waals forces?

Larger radii have weaker van der Waals interactions.

Can ionic radius change with coordination number?

Yes, it can vary with different coordination numbers in crystals.

How does ionic radius affect chemical properties?

Influences solubility, crystal structure, and melting points.

Is ionic radius the same in different compounds?

It can vary depending on the ionic environment.

Do ions always have different radii than their neutral atoms?

Yes, due to changes in electron count and structure.
About Author
Written by
Harlon Moss
Harlon is a seasoned quality moderator and accomplished content writer for Difference Wiki. An alumnus of the prestigious University of California, he earned his degree in Computer Science. Leveraging his academic background, Harlon brings a meticulous and informed perspective to his work, ensuring content accuracy and excellence.
Edited by
Aimie Carlson
Aimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.

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