---
title: "The potential energy curve for the interaction between isolated gaseous \\(\\text{Na}^+\\) and \\(\\text{Cl}^-\\) ions as a function of internuclear distance is represented by the solid curve in the graph above. A student predicts how substituting \\(\\text{K}^+\\) in place of \\(\\text{Na}^+\\) alters this curve. Which of the labeled curves best represents the potential energy curve for the interaction between isolated gaseous \\(\\text{K}^+\\) and \\(\\text{Cl}^-\\) ions, and what is the correct justification?"
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url: "https://nerd-notes.com/ubq/123547/"
date_modified: "2026-09-28T12:00:06+00:00"
---

# The potential energy curve for the interaction between isolated gaseous \(\text{Na}^+\) and \(\text{Cl}^-\) ions as a function of internuclear distance is represented by the solid curve in the graph above. A student predicts how substituting \(\text{K}^+\) in place of \(\text{Na}^+\) alters this curve. Which of the labeled curves best represents the potential energy curve for the interaction between isolated gaseous \(\text{K}^+\) and \(\text{Cl}^-\) ions, and what is the correct justification?

The potential energy curve for the interaction between isolated gaseous \(\text{Na}^+\) and \(\text{Cl}^-\) ions as a function of internuclear distance is represented by the solid curve in the graph above. A student predicts how substituting \(\text{K}^+\) in place of \(\text{Na}^+\) alters this curve. Which of the labeled curves best represents the potential energy curve for the interaction between isolated gaseous \(\text{K}^+\) and \(\text{Cl}^-\) ions, and what is the correct justification?

![A grayscale potential energy plot with horizontal axis labeled Internuclear Distance (\(\text{pm}\)) and vertical axis labeled Potential Energy (\(\text{kJ/mol}\)). A horizontal dashed line indicates zero potential energy in the upper portion of the graph. A solid curve labeled \(\text{NaCl(g)}\) starts at high positive energy at small internuclear distance, drops steeply to a potential energy minimum at intermediate distance \(r_1\) and negative energy \(E_1\), and then asymptotically approaches the zero line as distance increases. Four labeled candidate curves appear with distinct minima: Curve W (dashed line) has its minimum to the left of and below \((r_1, E_1)\); Curve X (dotted line) has its minimum to the left of and above \((r_1, E_1)\); Curve Y (dash-dotted line) has its minimum to the right of and above \((r_1, E_1)\); Curve Z (short-dashed line) has its minimum to the right of and below \((r_1, E_1)\). All curves asymptotically approach zero potential energy at large internuclear distance. No other curves, data points, shaded regions, or annotations appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1790596806-MoDOsq.jpg)

- **A.** Curve W, because \(\text{K}^+\) has a greater nuclear charge than \(\text{Na}^+\), exerting a stronger electrostatic pull on \(\text{Cl}^-\) that shortens the equilibrium bond distance and deepens the potential energy well.
- **B.** Curve X, because \(\text{K}^+\) has more protons that pull the \(\text{Cl}^-\) ion closer, but increased electron-electron repulsion between core electrons results in a shallower potential energy well.
- **C.** Curve Y, because \(\text{K}^+\) has an additional occupied electron shell compared to \(\text{Na}^+\), resulting in a larger ionic radius and a weaker Coulombic attraction that yields a shallower potential energy well.
- **D.** Curve Z, because \(\text{K}^+\) has a larger ionic radius than \(\text{Na}^+\), but its greater number of electrons increases polarizability, strengthening dispersion interactions and deepening the potential energy well.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/123547/*