---
title: "A student models the electrostatic potential energy between two isolated gaseous ions as a function of internuclear separation distance, as shown in the graph. The solid curve represents Pair 1, which consists of a \\(\\text{K}^+\\) ion and a \\(\\text{Cl}^-\\) ion. The dashed curve represents Pair 2, which consists of a different pair of interacting gaseous ions. Which of the following identifies the ions in Pair 2 and provides the correct justification?"
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url: "https://nerd-notes.com/ubq/120197/"
date_modified: "2026-08-21T16:03:03+00:00"
---

# A student models the electrostatic potential energy between two isolated gaseous ions as a function of internuclear separation distance, as shown in the graph. The solid curve represents Pair 1, which consists of a \(\text{K}^+\) ion and a \(\text{Cl}^-\) ion. The dashed curve represents Pair 2, which consists of a different pair of interacting gaseous ions. Which of the following identifies the ions in Pair 2 and provides the correct justification?

A student models the electrostatic potential energy between two isolated gaseous ions as a function of internuclear separation distance, as shown in the graph. The solid curve represents Pair 1, which consists of a \(\text{K}^+\) ion and a \(\text{Cl}^-\) ion. The dashed curve represents Pair 2, which consists of a different pair of interacting gaseous ions. Which of the following identifies the ions in Pair 2 and provides the correct justification?

![A line graph shows potential energy in \(\text{kJ/mol}\) on the vertical y-axis versus internuclear distance in \(\text{pm}\) on the horizontal x-axis. The y-axis has a horizontal dashed reference line at \(0\text{ kJ/mol}\), with positive values above and negative values below. The x-axis extends from \(0\text{ pm}\) to \(500\text{ pm}\). A solid curve labeled Pair 1 starts at a high positive potential energy near \(150\text{ pm}\), decreases steeply to cross \(0\text{ kJ/mol}\) at \(200\text{ pm}\), reaches a minimum at \(r = 280\text{ pm}\) with a potential energy of \(-430\text{ kJ/mol}\), and then rises smoothly to asymptotically approach \(0\text{ kJ/mol}\) as distance increases. A dashed curve labeled Pair 2 begins to the left of the solid curve, drops steeply to cross \(0\text{ kJ/mol}\) at \(160\text{ pm}\), reaches a deeper minimum at \(r = 220\text{ pm}\) with a potential energy of \(-580\text{ kJ/mol}\), and rises asymptotically toward \(0\text{ kJ/mol}\). No other curves, labels, text, or annotations appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1787328183-rGQF8B.jpg)

- **A.** Pair 2 is \(\text{Rb}^+\text{–Br}^-\), because \(\text{Rb}^+\) and \(\text{Br}^-\) have more occupied electron shells than \(\text{K}^+\) and \(\text{Cl}^-\), resulting in greater electron shielding and a deeper potential energy well.
- **B.** Pair 2 is \(\text{K}^+\text{–Br}^-\), because \(\text{Br}^-\) has a greater nuclear charge than \(\text{Cl}^-\), which exerts a stronger Coulombic attraction that shifts the energy minimum to a shorter internuclear distance.
- **C.** Pair 2 is \(\text{Na}^+\text{–F}^-\), because \(\text{Na}^+\) and \(\text{F}^-\) have fewer occupied electron shells than \(\text{K}^+\) and \(\text{Cl}^-\), leading to smaller ionic radii and a stronger Coulombic attraction at a shorter internuclear distance.
- **D.** Pair 2 is \(\text{Na}^+\text{–F}^-\), because \(\text{F}\) has a higher electronegativity than \(\text{Cl}\), which increases the covalent character of the interaction and deepens the potential energy well.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/120197/*