Anand Classes explains Ionic radius is the measure of the size of an ion, defined as the distance from the nucleus of the ion to the outermost electron shell. When an atom loses electrons to form a cation, the number of protons exceeds the number of electrons, pulling the remaining electrons closer to the nucleus and reducing the ionic radius. In contrast, when an atom gains electrons to form an anion, the increased electron–electron repulsion expands the outer shell, increasing the ionic radius. Understanding ionic radius trends across a period and down a group is important for predicting chemical reactivity, bond strength, and physical properties, making it a key topic for JEE, NEET, and CBSE Board examinations.
Table of Contents
Define Ionic Radius
The ionic radius refers to the effective distance from the centre of the nucleus of an ion up to the point where it has an influence in an ionic bond.
Ions are formed when atoms gain or lose electrons:
- Cations: Positively charged ions formed by the loss of electrons (e.g., Na⁺, Mg²⁺).
- Anions: Negatively charged ions formed by the gain of electrons (e.g., Cl⁻, O²⁻).
Measurement of Ionic Radius
The most common way to measure ionic radius is through X-ray crystallography, which gives the distance between the nuclei of adjacent positive and negative ions in an ionic crystal.
If the ions are treated as spheres, the internuclear distance can be expressed as:
Internuclear Distance = rcation + ranion
Example:
In sodium chloride (NaCl), the internuclear distance between Na+ and Cl– is 276 pm.
- This value is the sum of the ionic radii of Na+ and Cl–.
- If the radius of one ion is known, the other can be calculated by subtraction.
Determining Ionic Radii
Since measuring the radius of both ions directly is challenging, scientists fix the radius of at least one standard ion (commonly F⁻ or O²⁻) and calculate others accordingly.
- The Pauling method is the most widely accepted approach for this purpose.
General Trends in Ionic Radius
From studies, we observe two important patterns:
- Cations are Smaller than Parent Atoms
- When an atom loses electrons to form a cation, the loss of an electron shell and stronger attraction between the nucleus and remaining electrons cause the radius to shrink.
- Example: Na (186 pm) → Na⁺ (102 pm)
- Anions are Larger than Parent Atoms
- When an atom gains electrons to form an anion, electron-electron repulsion increases, and the nuclear pull per electron decreases, causing the radius to expand.
- Example: Cl (99 pm) → Cl⁻ (181 pm)
Frequently Asked Questions (FAQs) on Ionic Radius
Q1. What is ionic radius?
The ionic radius is the effective distance from the nucleus of an ion to the point up to which it exerts an influence in an ionic bond.
Q2. How is ionic radius measured?
Ionic radii are generally determined using X-ray diffraction studies, which measure the distance between nuclei of adjacent ions in ionic crystals.
Q3. Why is the radius of a cation smaller than its parent atom?
When an atom loses electrons to form a cation, it loses an outer electron shell and experiences increased effective nuclear charge, pulling the remaining electrons closer to the nucleus.
Q4. Why is the radius of an anion larger than its parent atom?
When an atom gains electrons to form an anion, electron-electron repulsion increases and the nuclear pull on each electron decreases, resulting in a larger size.
Q5. Can ionic radius be the same for cations and anions of the same element?
No. Cations are always smaller, and anions are always larger than their respective parent atoms due to differences in electron configuration and effective nuclear charge.
Q6. What is Pauling’s method of determining ionic radius?
Pauling’s method is a standard approach that fixes the radius of at least one ion as a reference, allowing calculation of other ionic radii by subtracting from the measured internuclear distances.
Q7. How does ionic radius affect physical properties of compounds?
Ionic radius influences properties like lattice energy, melting point, and solubility — smaller ions generally form stronger ionic bonds and higher melting point compounds.
Do You Know? – Ionic Radius Facts
- 🔹 Francium is the largest alkali metal in terms of ionic radius, while Fluoride (F⁻) is one of the largest anions.
- 🔹 Lithium ion (Li⁺) is so small that it can easily move through crystal lattices, which is why lithium batteries are highly efficient.
- 🔹 Cations of transition metals often shrink more than expected due to d-orbital contraction.
- 🔹 The oxide ion (O²⁻) has a much larger radius than oxygen atom because it gains two electrons, causing strong repulsion.
- 🔹 Lattice energy increases sharply as ionic radii decrease — smaller ions form stronger ionic bonds.
- 🔹 In the periodic table, ionic radius increases down a group because new electron shells are added.
- 🔹 Isoelectronic species (ions with the same number of electrons) have different radii due to differences in nuclear charge. For example: O²⁻ > F⁻ > Na⁺ > Mg²⁺.
- 🔹 The concept of effective nuclear charge explains why cations shrink and anions expand compared to their neutral atoms.
📚 Buy Study Material & Join Our Coaching
For premium study materials specially designed for JEE, NEET, NDA, and CBSE/ICSE Classes, visit our official study material portal:
👉 https://anandclasses.net.in/
To enroll in our offline or online coaching programs, visit our coaching center website:
👉 https://anandclasses.co.in/
📞 Call us directly at: +91-94631-38669
💬 WhatsApp Us Instantly
Need quick assistance or want to inquire about classes and materials?
📲 Click below to chat instantly on WhatsApp:
👉 Chat on WhatsApp
🎥 Watch Video Lectures
Get access to high-quality video lessons, concept explainers, and revision tips by subscribing to our official YouTube channel:
👉 Neeraj Anand Classes – YouTube Channel
