Question

Cesium has the smallest ionization energy of all elements ( 376 \(\mathrm{kJ} / \mathrm{mol}\) ), and chlorine has the most negative electron affinity \((-349 \mathrm{~kJ} / \mathrm{mol}) .\) Will a cesium atom transfer an electron to a chlorine atom to form isolated \(\mathrm{Cs}^{+}(\mathrm{g})\) and \(\mathrm{Cl}^{-}(g)\) ions? Explain.

Short answer

No, the electron transfer is not spontaneous as it requires 27 kJ/mol of energy.

Step-by-step solution

Identify the Significant Values

Cesium has the smallest ionization energy of 376 \(\mathrm{kJ}/\mathrm{mol}\), meaning it requires this amount of energy to lose an electron. Chlorine has the most negative electron affinity of -349 \(\mathrm{kJ}/\mathrm{mol}\), indicating it releases this amount of energy when gaining an electron.

Calculate Energy Change for Electron Transfer

To determine if an electron transfer is feasible, calculate the net energy change in forming the ions. The energy change is given by \[ \Delta E = \text{Ionization Energy of Cs} + \text{Electron Affinity of Cl} \] Substituting the given values, we have \[ \Delta E = 376 \ \mathrm{kJ}/\mathrm{mol} - 349 \ \mathrm{kJ}/\mathrm{mol} = 27 \ \mathrm{kJ}/\mathrm{mol} \]

Analyze the Energy Change

The calculated \(\Delta E\) is 27 \(\mathrm{kJ}/\mathrm{mol}\). Since this is a positive value, it indicates that the transfer of an electron from cesium to chlorine requires additional energy rather than releasing energy. This means the process is not energetically favorable in forming isolated \(\mathrm{Cs}^{+} (g)\) and \(\mathrm{Cl}^{-} (g)\) ions spontaneously.

Key concepts

Electron Affinity

Electron affinity is a measure of the tendency of an atom to gain an electron and form a negative ion. It represents the amount of energy released when an electron is added to a gaseous atom. This concept is crucial in understanding how atoms interact with one another, especially in forming ionic bonds.

Clorine is an atom with high electron affinity because when it gains an electron, it releases a substantial amount of energy. This is marked by a negative value, meaning energy is released rather than consumed in the process. High electron affinity suggests that the atom strongly attracts electrons, making it a potential candidate for accepting electrons from atoms with low ionization energy like cesium. Understanding electron affinity helps explain why certain elements tend to form anions more readily than others.

Cesium

Cesium is a fascinating element known for having the lowest ionization energy among all elements, measured at 376 \text{kJ}/\text{mol}. This low energy requirement makes it easy for cesium to lose an electron and form a positive ion, \( \text{Cs}^+ \).

Because of cesium’s unique positioning in the periodic table, it acts as a highly reactive metal. The low ionization energy is because the lone valence electron in cesium is weakly bound and far from the nucleus, due to cesium's large atomic size. Thus, cesium readily participates in reactions where electron loss is advantageous, especially with nonmetals that tend to gain electrons easily.

• High reactivity due to large atomic radius
• Low ionization energy facilitates electron loss
• Forms \( \text{Cs}^+ \) ions with relative ease

Chlorine

Chlorine is a halogen known for its high electron affinity, with a notable value of \(-349 \, \mathrm{kJ}/\mathrm{mol}\). This high electron affinity indicates that chlorine atoms strongly desire to gain an electron in order to achieve a stable electronic configuration resembling that of noble gases.

When chlorine gains an electron, it forms \( \text{Cl}^- \) ions and releases energy, demonstrating why it strongly attracts electrons in chemical reactions. This is particularly significant in reactions with metals such as cesium, where chlorine can potentially accept electrons.

• High electron affinity implies strong electron attraction
• Tends to form \( \text{Cl}^- \) ions
• Participates in forming ionic bonds with metals

Electron Transfer

Electron transfer involves the movement of an electron from one atom to another, significantly impacting the formation of ions and ionic bonds. This process is vital in generating compounds with different properties compared to their parent atoms.

In the case of cesium and chlorine, electron transfer would theoretically involve cesium giving up an electron and chlorine accepting it. However, when calculating the energy changes, it is found that transferring an electron from cesium to chlorine in isolation results in a positive energy change of 27 \( \mathrm{kJ}/\mathrm{mol} \).

This positive energy change indicates that additional energy is required to make the transfer happen, making the spontaneous formation of \( \text{Cs}^+ \) and \( \text{Cl}^- \) ions energetically unfavorable under these conditions. Therefore, while cesium and chlorine engage in electron transfer easily in a reaction, doing so in gas phase isolation is not spontaneous.

• Involves electron donation and acceptance
• Depends on ionization energy and electron affinity
• Overall energy change determines feasibility