---
title: "To estimate steric interactions in a porous material, a student compares the electron-cloud radii of the species shown in the table.  | Species | Number of protons | Number of electrons | |—|—:|—:| | \\(\\text{S}^{2-}\\) | 16 | 18 | | \\(\\text{Ar}\\) | 18 | 18 | | \\(\\text{Ca}^{2+}\\) | 20 | 18 |  Which statement gives the correct ordering of electron-cloud radii from greatest to least and the best justification?"
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url: "https://nerd-notes.com/ubq/119404/"
date_modified: "2026-08-19T12:40:10+00:00"
---

# To estimate steric interactions in a porous material, a student compares the electron-cloud radii of the species shown in the table.

| Species | Number of protons | Number of electrons |
|—|—:|—:|
| \(\text{S}^{2-}\) | 16 | 18 |
| \(\text{Ar}\) | 18 | 18 |
| \(\text{Ca}^{2+}\) | 20 | 18 |

Which statement gives the correct ordering of electron-cloud radii from greatest to least and the best justification?

To estimate steric interactions in a porous material, a student compares the electron-cloud radii of the species shown in the table.

| Species | Number of protons | Number of electrons |
|---|---:|---:|
| \(\text{S}^{2-}\) | 16 | 18 |
| \(\text{Ar}\) | 18 | 18 |
| \(\text{Ca}^{2+}\) | 20 | 18 |

Which statement gives the correct ordering of electron-cloud radii from greatest to least and the best justification?

- **A.** \(\text{Ca}^{2+} > \text{Ar} > \text{S}^{2-}\), because the additional protons provide greater shielding of the outer electrons, allowing the electron cloud to expand.
- **B.** \(\text{Ca}^{2+} > \text{Ar} > \text{S}^{2-}\), because for the same electron configuration, a greater nuclear charge attracts the electrons more strongly.
- **C.** \(\text{S}^{2-} > \text{Ar} > \text{Ca}^{2+}\), because forming \(\text{S}^{2-}\) adds a new occupied principal energy level, whereas forming \(\text{Ca}^{2+}\) removes one.
- **D.** \(\text{S}^{2-} > \text{Ar} > \text{Ca}^{2+}\), because all three species have the same electron configuration, and increasing nuclear charge pulls the electron cloud closer to the nucleus.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/119404/*