Problem 71 - Chemistry The Molecular Science

Question

Give the electron configurations of these ions, and indicate which ones are isoelectronic. (a) \(\mathrm{Na}^{+}\) (b) \(\mathrm{Al}^{3+}\) (c) \(\mathrm{Cl}^{-}\)

Step-by-Step Solution

Answer

The ions \(\mathrm{Na}^{+}\) and \(\mathrm{Al}^{3+}\) are isoelectronic.

Step 1: Determine the Atomic Number and Neutral Atom Configuration

First, determine the atomic numbers for Na, Al, and Cl, which are 11, 13, and 17 respectively. Then, write the electron configurations for the neutral atoms: \(\mathrm{Na}: 1s^2 2s^2 2p^6 3s^1\); \(\mathrm{Al}: 1s^2 2s^2 2p^6 3s^2 3p^1\); \(\mathrm{Cl}: 1s^2 2s^2 2p^6 3s^2 3p^5\).

Step 2: Adjust for Charge to Find Ion Configuration

For ions, adjust the electron number based on the charge. Positive charges mean electrons are removed; negative charges mean electrons are added. \(\mathrm{Na}^{+}\) loses one electron, \(\mathrm{Al}^{3+}\) loses three electrons, and \(\mathrm{Cl}^{-}\) gains one electron.

Step 3: Write the Ion Configurations

Calculate the electron configurations of each ion: \(\mathrm{Na}^{+}: 1s^2 2s^2 2p^6\); \(\mathrm{Al}^{3+}: 1s^2 2s^2 2p^6\); \(\mathrm{Cl}^{-}: 1s^2 2s^2 2p^6 3s^2 3p^6 = 1s^2 2s^2 2p^6 3s^2\).

Step 4: Identify Isoelectronic Ions

Compare the electron configurations to determine which ions are isoelectronic. Isoelectronic ions have the same electron configuration. Here, \(\mathrm{Na}^{+}\) and \(\mathrm{Al}^{3+}\) are isoelectronic with each other, both having the configuration \(1s^2 2s^2 2p^6\). \(\mathrm{Cl}^{-}\) is not isoelectronic with them, as it has an additional electron in the 3rd shell.

Key Concepts

Isoelectronic Ions

Isoelectronic ions are ions that have the same number of electrons and hence the same electron configuration. This means they have similar electronic structures, which gives them some comparable chemical properties. To find if two ions are isoelectronic, you should:

• Determine the electron configuration of each ion.
• Check for matching electron configurations.

In the exercise, we see this with \( \mathrm{Na}^{+} \) and \( \mathrm{Al}^{3+} \) ions, both having the configuration \( 1s^2 2s^2 2p^6 \). Notice that despite the different elements, by gaining or losing electrons to form respective ions, they achieve the same electron configuration. This example shows how ions from different elements can become isoelectronic by adjusting their electron count through gaining or losing electrons. However, \( \mathrm{Cl}^{-} \) differs as it has an extra electron in the next shell, making its configuration \( 1s^2 2s^2 2p^6 3s^2 3p^6 \).

Neutral Atom Configuration

Before you can adjust for ion charges, it's crucial to know the electron configuration of the neutral atom. This is the starting point for understanding how electrons are arranged in an atom without any charge. For instance:

• \( \mathrm{Na} \) (sodium) has an atomic number of 11 with a neutral configuration of \( 1s^2 2s^2 2p^6 3s^1 \).
• \( \mathrm{Al} \) (aluminum) with atomic number 13 is \( 1s^2 2s^2 2p^6 3s^2 3p^1 \).
• \( \mathrm{Cl} \) (chlorine) has 17 electrons in its neutral state as \( 1s^2 2s^2 2p^6 3s^2 3p^5 \).

Knowing the neutral configurations helps us understand how and why atoms lose or gain electrons when forming ions. It sets the stage for making the necessary electronic adjustments needed to achieve stable, often full-shell configurations in ions.

Ion Charge Adjustment

Ion charge adjustment is the process of adding or removing electrons from a neutral atom to form a charged ion, based on the ion's charge. This adjustment is key to finding the correct electron configuration of the ion. Here's how it works:

• Positive ion charges (cations) mean electrons are lost. For example, \( \mathrm{Na}^{+} \) loses one electron, simplifying its configuration to \( 1s^2 2s^2 2p^6 \), and \( \mathrm{Al}^{3+} \) loses three electrons to have \( 1s^2 2s^2 2p^6 \).
• Negative ion charges (anions) indicate gained electrons. \( \mathrm{Cl}^{-} \) gains one electron, leading to a configuration \( 1s^2 2s^2 2p^6 3s^2 3p^6 \).

These electron changes not only stabilize the ion but also often result in the ion becoming isoelectronic with a noble gas, achieving a more stable electronic configuration. Adjusting for ion charge is crucial for understanding how ions will interact, bond, and why they form in the first place.