Problem 38 - A Complete Resource Book in Chemistry for JEE Main

Question

For electron affinity of halogens which of the following is correct? (a) \(\mathrm{F}>\mathrm{I}\) (b) \(\mathrm{F}>\mathrm{CI}\) (c) \(\mathrm{Br}>\mathrm{Cl}\) (d) \(\mathrm{Br}>\mathrm{F}\)

Step-by-Step Solution

Answer

Statement (a) \( \mathrm{F} > \mathrm{I} \) is correct.

Step 1: Understand Electron Affinity

Electron affinity is the energy change when an electron is added to a neutral atom in the gas phase to form a negative ion. For halogens, they have high electron affinities because they are one electron short of a full valence shell.

Step 2: Analyze Halogens Order

In the periodic table, as you move down the group of halogens from fluorine to iodine, the atomic size increases. Generally, electron affinity decreases as atomic size increases because the added electron is further from the nucleus.

Step 3: Compare Electron Affinity of F and I

Fluorine (F) has a higher electron affinity than iodine (I) because F is smaller and can more effectively attract an added electron compared to I. Thus, \( \mathrm{F} > \mathrm{I} \).

Step 4: Compare Electron Affinity of F and Cl

Even though chlorine (Cl) is larger than fluorine, Cl has a higher electron affinity than F due to less electron-electron repulsion in its larger size, making \( \mathrm{Cl} > \mathrm{F} \).

Step 5: Compare Electron Affinity of Br and Cl

Bromine (Br) is larger than chlorine, leading to lower electron affinity for Br compared to Cl. Thus, the statement \( \mathrm{Br} > \mathrm{Cl} \) is incorrect.

Step 6: Compare Electron Affinity of Br and F

Fluorine has a higher electron affinity than bromine due to its smaller size, despite Cl having a higher electron affinity than F due to specific energetic configurations. Therefore, \( \mathrm{Br} > \mathrm{F} \) is also incorrect.

Key Concepts

Halogens

Halogens are a fascinating group of elements found in Group 17 of the periodic table. These elements include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At).
They are known for their high reactivity and are just one electron short of achieving a full valence shell of electrons, giving them strong electron affinities. Characteristics of Halogens:

• High Reactivity: The need to gain one electron to fill their outer shell makes halogens very reactive, especially with alkali metals and alkaline earth metals.
• Diatomic Molecules: In their elemental form, halogens exist as diatomic molecules, such as F₂ or Cl₂.
• Varied Physical States: At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid.

The chemistry of halogens is deeply influenced by their high electron affinities. Since they need just one more electron to achieve a stable noble gas configuration, they tend to gain electrons readily, forming negative ions (halides). Understanding these traits helps to explain the powerful oxidizing abilities of halogens.

Periodic Table Trends

The periodic table is organized in a way that reveals significant trends in the properties of elements. One of these essential trends involves electron affinity, which varies across the table.
In the case of electron affinity:

• Across a Period: Electron affinity generally increases across a period from left to right. This is due to increasing nuclear charge with no significant increase in shielding, making it easier for the atom to attract additional electrons.
• Down a Group: Conversely, electron affinity typically decreases as you move down a group. This happens because each subsequent element down a group has an additional electron shell, increasing the distance between the nucleus and the outer electrons and often causing increased shielding.

However, there are exceptions to these trends. For example, the high electron affinity of chlorine compared to fluorine is unusual, partly due to less electron-electron repulsion in its relatively larger atomic size. Understanding these trends is crucial for predicting the behavior of elements in chemical reactions.

Atomic Size

Atomic size is a key concept tied closely to the properties of an element, including its electron affinity. It refers to the size of an atom, which can be visualized by the distance between the nucleus and the outermost electrons. Atomic size changes as you move across the periodic table. Factors Affecting Atomic Size:

• Within a Group: Atomic size increases as you move down a group, owing to the addition of electron shells.
• Across a Period: Atomic size decreases from left to right across a period. This occurs because of increased nuclear charge, which pulls electrons closer to the nucleus.

In the context of halogens, atomic size plays a crucial role in determining electron affinity. As seen with fluorine, despite its smaller size, its electron-electron repulsion affects its electron affinity, distinguishing it from chlorine’s slightly larger yet higher electron affinity. Understanding atomic size is essential for grasping how and why elements interact the way they do in chemical reactions.