---
title: "The first six successive ionization energies for an unknown Period 3 element, \\(\\text{X}\\), are provided in the table below.  | Ionization step | Ionization energy (\\(\\text{kJ/mol}\\)) | | :— | :— | | First (\\(IE_1\\)) | \\(1{,}012\\) | | Second (\\(IE_2\\)) | \\(1{,}907\\) | | Third (\\(IE_3\\)) | \\(2{,}914\\) | | Fourth (\\(IE_4\\)) | \\(4{,}964\\) | | Fifth (\\(IE_5\\)) | \\(6{,}274\\) | | Sixth (\\(IE_6\\)) | \\(21{,}267\\) |  Which of the following correctly identifies element \\(\\text{X}\\), predicts the chemical formula of the stable binary ionic compound formed between \\(\\text{X}\\) and magnesium, and provides the correct justification for the large increase between \\(IE_5\\) and \\(IE_6\\)?"
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url: "https://nerd-notes.com/ubq/120179/"
date_modified: "2026-08-21T16:02:50+00:00"
---

# The first six successive ionization energies for an unknown Period 3 element, \(\text{X}\), are provided in the table below.

| Ionization step | Ionization energy (\(\text{kJ/mol}\)) |
| :— | :— |
| First (\(IE_1\)) | \(1{,}012\) |
| Second (\(IE_2\)) | \(1{,}907\) |
| Third (\(IE_3\)) | \(2{,}914\) |
| Fourth (\(IE_4\)) | \(4{,}964\) |
| Fifth (\(IE_5\)) | \(6{,}274\) |
| Sixth (\(IE_6\)) | \(21{,}267\) |

Which of the following correctly identifies element \(\text{X}\), predicts the chemical formula of the stable binary ionic compound formed between \(\text{X}\) and magnesium, and provides the correct justification for the large increase between \(IE_5\) and \(IE_6\)?

The first six successive ionization energies for an unknown Period 3 element, \(\text{X}\), are provided in the table below.

| Ionization step | Ionization energy (\(\text{kJ/mol}\)) |
| :--- | :--- |
| First (\(IE_1\)) | \(1{,}012\) |
| Second (\(IE_2\)) | \(1{,}907\) |
| Third (\(IE_3\)) | \(2{,}914\) |
| Fourth (\(IE_4\)) | \(4{,}964\) |
| Fifth (\(IE_5\)) | \(6{,}274\) |
| Sixth (\(IE_6\)) | \(21{,}267\) |

Which of the following correctly identifies element \(\text{X}\), predicts the chemical formula of the stable binary ionic compound formed between \(\text{X}\) and magnesium, and provides the correct justification for the large increase between \(IE_5\) and \(IE_6\)?

- **A.** Element \(\text{X}\) is \(\text{Si}\), forming \(\text{Mg}_2\text{X}\), because the fourth ionization energy completely removes the \(3p\) subshell, causing the subsequent electrons to experience a greater effective nuclear charge.
- **B.** Element \(\text{X}\) is \(\text{S}\), forming \(\text{MgX}\), because six ionization energies are listed, indicating that all six valence electrons are removed before reaching a core noble-gas configuration.
- **C.** Element \(\text{X}\) is \(\text{P}\), forming \(\text{Mg}_3\text{X}_2\), because the first five electrons are removed from the \(n = 3\) valence shell, whereas the sixth electron is removed from the \(n = 2\) core shell, which experiences less electron shielding.
- **D.** Element \(\text{X}\) is \(\text{P}\), forming \(\text{Mg}_2\text{X}_3\), because the removal of the sixth electron requires pairing electrons in the \(2p\) subshell, resulting in increased electron-electron repulsion.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/120179/*