---
title: "Interionic Distance — AP Chem Definition & Exam Guide"
description: "Interionic distance is the cation-to-anion separation in an ionic solid. It's the r in Coulomb's law, so smaller ions mean stronger attraction and higher lattice energy."
canonical: "https://fiveable.me/ap-chem/key-terms/interionic-distance"
type: "key-term"
subject: "AP Chemistry"
unit: "Unit 2"
---

# Interionic Distance — AP Chem Definition & Exam Guide

## Definition

Interionic distance is the separation between a cation and an anion in an ionic solid, equal to the sum of their ionic radii; it is the distance variable in Coulomb's law, so a smaller interionic distance means stronger electrostatic attraction and greater lattice energy.

## What It Is

Interionic distance is the center-to-center separation between a [cation](/ap-chem/key-terms/cation "fv-autolink") and an anion sitting next to each other in an ionic crystal. You can estimate it by adding the two ionic radii together. A compound made of small ions, like MgO, has a short interionic distance. A compound made of big ions, like KBr, has a long one.

Why does [AP Chem](/ap-chem "fv-autolink") care about a distance? Because it's the *r* in Coulomb's law. The attractive force between opposite charges depends on the magnitudes of the charges and the distance between them, and force drops off fast as distance grows. The CED (essential knowledge 2.3.A.1) describes ionic solids as 3-D arrays arranged to maximize attraction and minimize repulsion, and interionic distance is one of the two levers (along with ion [charge](/ap-chem/unit-9 "fv-autolink")) that controls how strong those attractions are. Shorter distance means the ions are held more tightly, which shows up macroscopically as higher lattice energy and a higher melting point.

## Why It Matters

This term lives in Topic 2.3 (Structure of Ionic Solids) in [Unit 2](/ap-chem/unit-2 "fv-autolink") and supports learning objective AP Chem 2.3.A, which asks you to represent an [ionic solid](/ap-chem/key-terms/ionic-solid "fv-autolink") with a particulate model consistent with Coulomb's law and the ions' properties. In practice, that means drawing or interpreting diagrams where ion sizes and spacing actually match reality. Interionic distance is also half of every lattice energy comparison you'll make on the exam. When two compounds have the same ion charges, distance is the tiebreaker. One reassuring note from the CED exclusion statement is that you will never be asked to name specific crystal structures (no rock salt vs. cesium chloride geometry). The exam only wants the Coulomb's law reasoning.

## Connections

### [Coulomb's Law (Units 1-2)](/ap-chem/key-terms/coulombs-law)

Interionic distance is literally the r in Coulomb's law. The same charge-and-distance logic you used in [Unit 1](/ap-chem/unit-1 "fv-autolink") to explain ionization energy trends gets recycled in Unit 2 to explain why some ionic solids are held together more tightly than others.

### [Lattice Energy (Unit 2)](/ap-chem/key-terms/lattice-energy)

[Lattice energy](/ap-chem/key-terms/lattice-energy "fv-autolink") is the macroscopic payoff of interionic distance. Compare two compounds with the same charges, like MgCl₂ and SrCl₂, and the one with the smaller cation has the shorter distance and the larger lattice energy.

### [Ionic Radius (Units 1-2)](/ap-chem/key-terms/ionic-radius)

Interionic distance is built from [ionic radii](/ap-chem/key-terms/ionic-radius "fv-autolink"). Periodic trends from Unit 1 (radius increases down a group, cations are smaller than their parent atoms) tell you which compound has the shorter distance without any numbers.

### [Melting Point (Unit 2)](/ap-chem/key-terms/melting-point)

Melting an ionic solid means pulling ions apart against their Coulombic attraction. Shorter interionic distance means stronger attraction, so MgO melts far above NaCl, which melts above KBr.

## On the AP Exam

This concept shows up almost entirely as comparison and ranking questions. A typical MCQ gives you two or three ionic compounds and asks which has the highest lattice energy or melting point, like ranking MgO, NaCl, and KBr. Your job is a two-step check. First compare ion charges (the bigger factor), then compare interionic distance using ionic radii. Practice questions also hand you particulate models and ask which drawing is consistent with Coulomb's law, so make sure relative ion sizes and spacing in a diagram make sense. On FRQs, this is justification territory. Released free-response questions about ionic compounds (like the 2017 short FRQ on Mg(OH)₂) reward answers that explicitly cite charge magnitude and distance between ions, not vague claims like "the bond is stronger." Name both variables and say which one is doing the work in your comparison.

## Interionic Distance vs Ionic Radius

Ionic radius is the size of one ion by itself. Interionic distance is the gap between two neighboring ions in the crystal, roughly the cation radius plus the anion radius. You use radii (a Unit 1 periodic trend) to predict the distance, but only the distance plugs into Coulomb's law. Mixing them up usually leads to comparing one ion's size when the exam wants the sum.

## Key Takeaways

- Interionic distance is the separation between a cation and an anion in an ionic solid, estimated by adding the two ionic radii.
- It is the r in Coulomb's law, so a shorter interionic distance means stronger electrostatic attraction between the ions.
- When comparing lattice energies, check ion charges first because charge matters more, then use interionic distance as the tiebreaker.
- Shorter interionic distance translates to higher lattice energy and a higher melting point, which is why MgO melts at a much higher temperature than KBr.
- The AP exam will never ask you to name specific crystal structures; it only tests whether your particulate model and reasoning are consistent with Coulomb's law (LO 2.3.A).

## FAQs

### What is interionic distance in AP Chem?

It's the separation between a cation and an anion in an ionic solid, roughly the sum of the two ionic radii. It matters because it's the distance term in Coulomb's law, which controls lattice energy and melting point.

### Does interionic distance matter more than ion charge for lattice energy?

No. Charge is the stronger factor because Coulomb's law multiplies the charge magnitudes. That's why MgO (2+ and 2-) beats NaCl (1+ and 1-) easily. Use distance to break ties when the charges are the same, like MgCl₂ vs. SrCl₂.

### How is interionic distance different from ionic radius?

Ionic radius describes one ion's size; interionic distance is the gap between two neighboring ions, basically the cation radius plus the anion radius. You use radii trends from Unit 1 to predict the distance, but the distance is what goes into Coulomb's law.

### Do I need to memorize crystal structures for the AP Chem exam?

No. The CED's exclusion statement for Topic 2.3 says specific crystal structures will not be assessed. You only need to reason that ions arrange in a 3-D array maximizing attractions and minimizing repulsions, and apply Coulomb's law to charge and distance.

### Why does MgO have a higher melting point than KBr?

Two reasons stack up. Mg²⁺ and O²⁻ carry double the charge of K⁺ and Br⁻, and they're smaller ions, so the interionic distance is shorter. Both factors make the Coulombic attraction much stronger, so far more energy is needed to pull the lattice apart.

## Related Study Guides

- [2.3 Structure of Ionic Solids](/ap-chem/unit-2/structure-ionic-solids/study-guide/3khaTI6A3tdnaMTMJy2u)

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