Problem 8 - Chemistry An Atoms First Approach

Question

The second electron affinity values for both oxygen and sulfur are unfavorable (positive). Explain.

Step-by-Step Solution

Answer

The second electron affinity values for both oxygen and sulfur are unfavorable (positive) because adding a second electron to their already negatively charged ions (O⁻ and S⁻) increases the system's energy due to electron-electron repulsion. The incoming electron experiences strong repulsion from the negatively charged ion and is forced to occupy a higher energy orbital, making the process of gaining a second electron unfavorable.

Step 1: Understand electron affinity

Electron affinity is the energy that is released (exothermic) or absorbed (endothermic) when an electron is added to an atom to form a negative ion. It is a measure of how much an atom wants to gain an electron. The sign of electron affinity can be positive or negative, indicating a favorable (exothermic) or unfavorable (endothermic) process, respectively.

Step 2: Examine the electron configuration of oxygen and sulfur

Oxygen has an electron configuration of 1s² 2s² 2p^4, and sulfur has an electron configuration of 1s² 2s² 2p^6 3s² 3p^4. When both elements gain one electron, they form a negative ion with stable configurations (half-filled or fully filled p orbitals): Oxygen: 1s² 2s² 2p^5 (O⁻) and Sulfur: 1s² 2s² 2p^6 3s² 3p^5 (S⁻).

Step 3: Analyze the first electron affinity

The first electron affinity for oxygen and sulfur is negative (favorable). The energy is released when they gain one electron. This is because the incoming electron will occupy a stable position (half-filled or fully filled p orbitals), leading to a lower potential energy and increased stability.

Step 4: Explain why the second electron affinity is positive (unfavorable)

The second electron affinity for oxygen and sulfur is positive (unfavorable) because adding an additional electron requires energy. This is because the incoming electron will experience strong repulsion from the negatively charged ion (O⁻ or S⁻) that has already gained an electron. The additional electron will be forced to occupy a higher energy orbital, causing the system's energy to increase. Consequently, the process of gaining a second electron is unfavorable, and the second electron affinity is positive. In summary, the second electron affinity values for both oxygen and sulfur are unfavorable (positive) because adding a second electron to their already negatively charged ions results in an increase in energy due to electron-electron repulsion, which makes the process unfavorable.

Key Concepts

Electron Affinity Explained

Electron affinity is a term that describes the change in energy when an atom gains an electron to form an anion. In simple terms, it's like a measure of how much an atom 'wants' an extra electron. This process can either release energy, which we call exothermic, or absorb energy, known as endothermic.

For many elements, the first electron affinity is negative because adding the first electron usually leads to a more stable electronic arrangement, thus releasing energy. However, the second electron affinity is often positive, particularly for nonmetals like oxygen and sulfur. The positivity indicates that energy must be put in to add another electron against the electrostatic repulsion of the negatively charged ion formed after the first electron addition.

Understanding electron affinity is crucial for predicting how elements will interact, especially when they form ionic compounds. For instance, elements with high negative electron affinities are more likely to accept electrons and form anions during chemical reactions.

Electron Configuration and Stability

Understanding electron configuration is key to making sense of many chemical phenomena. The electron configuration tells us how the electrons are distributed among an atom's orbitals, which determines the atom's chemical behavior.

An important aspect of electron configurations is achieving a full or half-full set of p orbitals, which is energetically favorable and lends stability to the atom. The electron configuration of oxygen is 1s² 2s² 2p⁴, and for sulfur it's 1s² 2s² 2p⁶ 3s² 3p⁴. Once they gain one electron, they achieve a half-filled p orbital (for oxygen) or a filled p orbital (for sulfur), which is quite stable.

However, adding a second electron to these ions disrupts this stability and forces the electron into a more repulsive environment, further from the nucleus and among other negatively charged electrons. This need for additional energy to push the electron into a less stable position explains why the second electron affinities of oxygen and sulfur are unfavorable.

Exothermic and Endothermic Processes

These terms are essential for understanding energy changes in chemical reactions. An exothermic process is one that releases energy, often in the form of heat, making the surroundings warmer. Combustion is a classic example of an exothermic reaction.

On the flip side, an endothermic process absorbs energy from its surroundings, leading to a drop in temperature. Melting ice is an endothermic process, as it requires heat from the surroundings to change state from solid to liquid.

In the context of electron affinity, a negative value indicates an exothermic process, where energy is released when an atom gains an electron. The first electron affinities of oxygen and sulfur fall into this category. However, the second electron affinity involves an endothermic process; energy must be added to overcome electrostatic repulsion and add another electron to an already negative ion.