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:

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

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.