CI - is not isoelectronic with
Understanding Isoelectronic Species with Cl⁻
The question asks which of the given ions is not isoelectronic with the chloride ion, \(\text{Cl}^{-}\). Isoelectronic species are atoms or ions that have the same number of electrons. To answer this, we need to determine the number of electrons in \(\text{Cl}^{-}\) and then find the number of electrons in each of the given options.
Analyzing the Electron Count of \(\text{Cl}^{-}\)
The atomic number of Chlorine (\(\text{Cl}\)) is 17. This means a neutral chlorine atom has 17 protons and 17 electrons. The ion is \(\text{Cl}^{-}\), which has a charge of -1. This negative charge means it has gained one electron compared to the neutral atom.
Number of electrons in neutral \(\text{Cl}\) = 17
Number of electrons in \(\text{Cl}^{-}\) = Number of electrons in neutral \(\text{Cl}\) + 1 (for the -1 charge)
Number of electrons in \(\text{Cl}^{-}\) = \(17 + 1 = 18\) electrons.
So, the \(\text{Cl}^{-}\) ion has 18 electrons. Now we will check the electron count for each option.
Comparing \(\text{Cl}^{-}\) Electron Count with Options
Let's calculate the number of electrons for each given ion:
| Ion | Atomic Number | Charge | Number of Electrons | Calculation |
|---|---|---|---|---|
| \(\text{Cl}^{-}\) | 17 | -1 | 18 | \(17 + 1 = 18\) |
| \(\text{K}^{+}\) | 19 (Potassium) | +1 | 18 | \(19 - 1 = 18\) |
| \(\text{Mg}^{2+}\) | 12 (Magnesium) | +2 | 10 | \(12 - 2 = 10\) |
| \(\text{S}^{2-}\) | 16 (Sulfur) | -2 | 18 | \(16 + 2 = 18\) |
| \(\text{P}^{3-}\) | 15 (Phosphorus) | -3 | 18 | \(15 + 3 = 18\) |
From the table, we can see the number of electrons for each ion:
- \(\text{Cl}^{-}\) has 18 electrons.
- \(\text{K}^{+}\) has 18 electrons.
- \(\text{Mg}^{2+}\) has 10 electrons.
- \(\text{S}^{2-}\) has 18 electrons.
- \(\text{P}^{3-}\) has 18 electrons.
An ion is isoelectronic with \(\text{Cl}^{-}\) if it also has 18 electrons. Comparing the electron counts, we find that \(\text{K}^{+}\), \(\text{S}^{2-}\), and \(\text{P}^{3-}\) all have 18 electrons, just like \(\text{Cl}^{-}\). The \(\text{Mg}^{2+}\) ion, however, has only 10 electrons.
Conclusion: Identifying the Non-Isoelectronic Ion
Since \(\text{Mg}^{2+}\) has 10 electrons, which is a different number than the 18 electrons in \(\text{Cl}^{-}\), \(\text{Mg}^{2+}\) is not isoelectronic with \(\text{Cl}^{-}\).
Revision Table: Key Concepts for Isoelectronic Species
| Concept | Explanation | Example |
|---|---|---|
| Isoelectronic Species | Atoms or ions with the same number of electrons. | \(\text{Na}^{+}\) and \(\text{F}^{-}\) (both have 10 electrons) |
| Electron Count Calculation | Atomic Number - Charge (for cations); Atomic Number + Charge (for anions). | \(\text{Ca}^{2+}\): \(20 - 2 = 18\) electrons; \(\text{O}^{2-}\): \(8 + 2 = 10\) electrons |
| Noble Gas Configuration | Many ions are isoelectronic with noble gases (He, Ne, Ar, Kr, etc.) as this configuration is very stable. | \(\text{Cl}^{-}\), \(\text{K}^{+}\), \(\text{Ca}^{2+}\) are isoelectronic with Argon (18 electrons). \(\text{Mg}^{2+}\), \(\text{Na}^{+}\), \(\text{F}^{-}\) are isoelectronic with Neon (10 electrons). |
Additional Information: Electron Configurations and Stability
The concept of isoelectronic species is important because ions often form by gaining or losing electrons to achieve a stable electron configuration, usually that of a noble gas. The noble gases have a full outermost electron shell, making them very unreactive. Ions like \(\text{Cl}^{-}\), \(\text{K}^{+}\), \(\text{S}^{2-}\), and \(\text{P}^{3-}\) are all isoelectronic with the noble gas Argon (\(\text{Ar}\)), which has 18 electrons (\(1\text{s}^2 2\text{p}^6 3\text{s}^2 3\text{p}^6\)). The \(\text{Mg}^{2+}\) ion, with 10 electrons (\(1\text{s}^2 2\text{s}^2 2\text{p}^6\)), is isoelectronic with the noble gas Neon (\(\text{Ne}\)). Achieving these noble gas configurations leads to increased stability for the ions.
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