---
title: "The first ionization energies and ground-state valence electron configurations for four Period 2 elements are shown in the table below.  | Element | Valence Electron Configuration | First Ionization Energy (\\(\\text{kJ/mol}\\)) | | :— | :— | :— | | \\(\\text{Be}\\) | \\(2s^2\\) | \\(899\\) | | \\(\\text{B}\\) | \\(2s^2\\,2p^1\\) | \\(801\\) | | \\(\\text{N}\\) | \\(2s^2\\,2p^3\\) | \\(1402\\) | | \\(\\text{O}\\) | \\(2s^2\\,2p^4\\) | \\(1314\\) |  Although first ionization energy generally increases across a period with increasing nuclear charge, both \\(\\text{B}\\) and \\(\\text{O}\\) have lower first ionization energies than the preceding element. Which of the following statements provides the correct justification for both observed anomalies based on subshell electron configurations?"
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url: "https://nerd-notes.com/ubq/120175/"
date_modified: "2026-08-21T16:02:47+00:00"
---

# The first ionization energies and ground-state valence electron configurations for four Period 2 elements are shown in the table below.

| Element | Valence Electron Configuration | First Ionization Energy (\(\text{kJ/mol}\)) |
| :— | :— | :— |
| \(\text{Be}\) | \(2s^2\) | \(899\) |
| \(\text{B}\) | \(2s^2\,2p^1\) | \(801\) |
| \(\text{N}\) | \(2s^2\,2p^3\) | \(1402\) |
| \(\text{O}\) | \(2s^2\,2p^4\) | \(1314\) |

Although first ionization energy generally increases across a period with increasing nuclear charge, both \(\text{B}\) and \(\text{O}\) have lower first ionization energies than the preceding element. Which of the following statements provides the correct justification for both observed anomalies based on subshell electron configurations?

The first ionization energies and ground-state valence electron configurations for four Period 2 elements are shown in the table below.

| Element | Valence Electron Configuration | First Ionization Energy (\(\text{kJ/mol}\)) |
| :--- | :--- | :--- |
| \(\text{Be}\) | \(2s^2\) | \(899\) |
| \(\text{B}\) | \(2s^2\,2p^1\) | \(801\) |
| \(\text{N}\) | \(2s^2\,2p^3\) | \(1402\) |
| \(\text{O}\) | \(2s^2\,2p^4\) | \(1314\) |

Although first ionization energy generally increases across a period with increasing nuclear charge, both \(\text{B}\) and \(\text{O}\) have lower first ionization energies than the preceding element. Which of the following statements provides the correct justification for both observed anomalies based on subshell electron configurations?

- **A.** For \(\text{B}\), the electron is removed from a higher-energy \(2p\) subshell that is partially shielded by the \(2s\) electrons; for \(\text{O}\), the electron is removed from an expanded \(3s\) subshell located farther from the nucleus.
- **B.** For \(\text{B}\), the electron is removed from a doubly occupied orbital where electron-electron repulsion lowers the ionization energy; for \(\text{O}\), the electron is removed from a newly occupied \(2p\) subshell that is farther from the nucleus.
- **C.** For \(\text{B}\), the electron is removed from a \(2p\) subshell that experiences less nuclear charge than \(\text{Be}\); for \(\text{O}\), the electron is removed from a half-filled \(2p\) subshell that has lower binding energy.
- **D.** For \(\text{B}\), the electron is removed from a higher-energy \(2p\) subshell shielded by the filled \(2s\) subshell; for \(\text{O}\), the electron is removed from a doubly occupied \(2p\) orbital where electron-electron repulsion facilitates removal.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/120175/*