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

The correct order of ascending electron affinity is Se < S < O, which corresponds to option (c).

Step 1: Understanding Electron Affinity

Electron affinity refers to the amount of energy released when an atom in the gaseous state accepts an electron to form an anion. Elements in the same group usually show trends where electron affinity may decrease down a group due to the increasing energy levels that electrons reside in.

Step 2: Knowing Periodic Trends within Group 16

Sulfur (S), Oxygen (O), and Selenium (Se) all belong to Group 16 of the periodic table. Generally, as we move from top to bottom in a group, the electron affinity decreases. However, in some cases, elements in the second period (e.g., oxygen) might have slightly lower electron affinities than elements in the third period (e.g., sulfur) due to electron repulsion in smaller atoms.

Step 3: Comparing Electron Affinities of S, O, and Se

In Group 16, we usually expect sulfur to have a higher electron affinity than selenium, but oxygen has distinctively lower electron affinity than sulfur due to its smaller size and increased electron-electron repulsion. Hence the order should be: Se < S < O, which stands out as an anomaly but holds due to intershell repulsions in oxygen.

Step 4: Identifying the Correct Order

According to the typical trends modified by inter-personal electron repulsions in second-period elements, the correct order of ascending electron affinity is: Se < S < O, which matches option (c).

Key Concepts

Periodic Trends

Periodic trends are repetitive patterns that occur in the periodic table. One key trend is electron affinity, which generally increases across a period and decreases down a group. This means, elements to the right of the periodic table, such as halogens, tend to attract electrons more strongly. As we move down a group, adding energy levels increases the atomic radius.

This makes it harder for the nucleus to attract additional electrons, resulting in a decrease in electron affinity. In summary, the periodic trends of elements are crucial to understanding many properties, including electron affinity. Knowing these trends helps predict how atoms will behave when gaining, losing, or sharing electrons.

• Electron affinity generally increases across a period from left to right.
• Electron affinity generally decreases down a group from top to bottom.

Group 16 Elements

Group 16 on the periodic table is known as the chalcogens, and it includes elements such as oxygen (O), sulfur (S), and selenium (Se). These elements have six electrons in their outermost shell, making them two electrons short of a full octet. Their positioning in the periodic table influences their chemistry, including their electron affinity. Oxygen, being in the second period, is smaller in size.

This causes a greater electron repulsion amongst its own valence electrons, sometimes resulting in lower effective electron affinity compared to its heavier group members like sulfur and selenium. Generally, sulfur tends to show higher electron affinity than selenium.

• The group is characterized by having six valence electrons.
• Second-period members, like oxygen, might display anomalies in expected electron affinity due to electron-electron repulsions.

Anion Formation

Anion formation is a fundamental concept in understanding electron affinity. Anions are negatively charged ions formed when an atom gains one or more electrons. The formation of an anion releases energy, which is why electron affinity is often expressed as a negative value reflecting this release.

In the context of Group 16 elements, sulfur and selenium form anions by gaining electrons more easily compared to oxygen, due to fewer repulsions in their larger atomic structures. Understanding the order in which elements form anions, such as Se < S < O with respect to electron affinity, underlines the concept of anion stability and formation tendencies in Group 16.

• Anions are negatively charged ions formed by the gain of electrons.
• The energy released during anion formation is a crucial aspect of electron affinity.