Problem 46 - A Complete Resource Book in Chemistry for JEE Main

Question

Arrange \(\mathrm{S}, \mathrm{O}\) and Se in the ascending order of electron affinity. (a) \(\mathrm{S}<\mathrm{Se}<\mathrm{O}\) (b) \(\mathrm{Se}<\mathrm{O}<\mathrm{S}\) (c) \(\mathrm{Se}<\mathrm{S}<\mathrm{O}\) (d) \(\mathrm{S}<\mathrm{O}<\mathrm{Se}\)

Step-by-Step Solution

Answer

(c) \(\mathrm{Se} < \mathrm{S} < \mathrm{O}\)

Step 1: Understand Electron Affinity

Electron affinity is the amount of energy released when an electron is added to a neutral atom in the gaseous state. A higher electron affinity value indicates a greater tendency to gain an electron.

Step 2: Analyze Periodic Trends

In general, electron affinity increases across a period and decreases down a group. This is because elements across a period tend to have similar electron configurations that are closer to achieving a full valence shell, while going down a group, the additional electron shells reduce the nucleus' effective pull on added electrons.

Step 3: Evaluate Group 16 Elements

Oxygen (O), Sulfur (S), and Selenium (Se) are all in Group 16 of the periodic table. Oxygen is in the second period, while sulfur is in the third period and selenium is in the fourth period. Based on periodic trends, we expect oxygen to have higher electron affinity than sulfur and selenium.

Step 4: Consider Exception for Oxygen

Although oxygen generally has high electron affinity, it is an exception in its group, as it has a slightly lower electron affinity than sulfur due to increased electron-electron repulsions in its smaller p orbital.

Step 5: Arrange Elements by Observed Electron Affinity

Taking into account the periodic trends and exceptions, the correct order for electron affinity, from lowest to highest, is selenium (Se), sulfur (S), and then oxygen (O). So the correct arrangement is Se < S < O.

Key Concepts

Electron Affinity

Electron affinity is a crucial concept in chemistry. It measures the energy change when an electron is added to a neutral atom, forming a negative ion. In essence, it's the tendency of an atom to accept an additional electron. A higher value means the atom releases more energy, showing a stronger inclination to gain more electrons.
Typically:

• A high electron affinity indicates a strong desire to gain electrons.
• A low electron affinity means less energy is released during electron gain.

Electron affinity is often confused with electronegativity, but where electronegativity involves attraction in a bond, electron affinity focuses solely on energy change when gaining an electron.

Group 16 Elements

Group 16 elements of the periodic table include oxygen (21"o()), sulfur (22"s()), selenium (22"ce()), and others down this group. These are part of the chalcogens family and are known for their varied properties and applications.
Here are some noteworthy characteristics:

• Group 16 elements have six electrons in their outermost shell, giving them two fewer than a complete octet.
• Their ability to gain two electrons to fill this shell influences their affinity for electrons.
• This group contains elements across different periods, contributing to varied affinities and atomic behaviors.

These elements display ascending atomic numbers and mass as you move down the group, affecting their physical and chemical properties.

Periodic Trends

Periodic trends are reliable patterns on the periodic table that help predict element behavior. When it comes to electron affinity, there are a few general trends to consider:

• Across a period, electron affinity tends to increase from left to right.
• Down a group, the affinity generally decreases.

This is because elements on the right side of the table tend to desire a full valence shell. However, as you move down a group, additional electron shells provide more shielding and distance from the nucleus, reducing effective nuclear charge and affinity.
These trends simplify predicting element characteristics but do come with exceptions.

Exception for Oxygen

While oxygen is expected to have high electron affinity due to its position on the periodic table, it is an exception among the Group 16 elements.
The anomaly lies in electron-electron repulsion within oxygen's relatively small p orbital. This repulsion slightly lowers its affinity compared to what might be expected in comparison to sulfur, even though oxygen is higher in the period. This means although oxygen typically has a high draw towards electrons, its small atomic radius leads to increased repulsions when additional electrons are introduced.
Understanding such exceptions is crucial for in-depth chemical analysis and predicting element behavior correctly across different contexts.