Problem 65 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is the correct relation with regard to the electron afinity of halogen? (a) \(\mathrm{F}>\mathrm{Cl}\) (b) \(\mathrm{F}>\mathrm{I}\) (c) \(\mathrm{Br}>\mathrm{F}\) (d) \(\mathrm{Br}>\mathrm{Cl}\)

Step-by-Step Solution

Answer

The correct answer is (b) \( \mathrm{F} > \mathrm{I} \).

Step 1: Understanding Electron Affinity

Electron affinity is the amount of energy released when an electron is added to a neutral atom in the gaseous state. Halogens are known for having high electron affinities because they are one electron away from having a full valence shell.

Step 2: Trend of Electron Affinity in Halogens

In the halogen group (Group 17), electron affinity generally increases up the group, meaning that fluorine () should have the highest electron affinity, followed by chlorine (), bromine (), and iodine (). However, fluorine is an exception due to repulsion in its small-sized atom.

Step 3: Comparing Fluorine and Chlorine

Normally, the smaller the element, the higher the electron affinity. However, chlorine () actually has a higher electron affinity than fluorine () due to the electron-electron repulsion in fluorine's small 2p orbital.

Step 4: Checking Each Option

(a)  > ; this is incorrect because chlorine has higher electron affinity than fluorine. (b)  > ; this is correct because fluorine has higher electron affinity than iodine. (c)  > ; this is incorrect as fluorine is higher than bromine. (d)  > ; this is incorrect as chlorine is greater than bromine.

Step 5: Selecting the Correct Answer

Based on the understanding of electron affinities, option (b)  >  is the correct relation because fluorine has a higher electron affinity than iodine.

Key Concepts

Halogens

Halogens are a fascinating group of elements found in Group 17 of the periodic table. This group includes fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These elements are known for their high reactivity and distinct characteristics. One notable feature of halogens is that they are just one electron short of a complete valence shell, meaning they are eager to gain an electron and achieve a stable electronic configuration.

• Fluorine is the most electronegative and reactive element in this group.
• Chlorine is widely used in pharmaceuticals and as a disinfectant.
• Bromine is typically found in flame retardants and certain types of medication.
• Iodine is essential in biology, used in nutrition and medical imaging.

The properties of these elements are greatly influenced by their atomic structure and the number of electrons in their outer shell.

Periodic Trends

Periodic trends describe predictable patterns observable throughout the elements in the periodic table. Understanding these trends can help anticipate the chemical behavior and properties of different elements, such as electron affinity, atomic size, and electronegativity. As we move across the periodic table from left to right, we typically observe:

• Increasing electronegativity and ionization energies.
• Decreasing atomic radii as electrons are added to the same energy level.

For halogens, one specific trend is electron affinity, which often increases as you move up in a group. This results from smaller atoms having valence electrons closer to the nucleus, which allows them to more effectively attract additional electrons. While fluorine is expected to have the highest electron affinity due to these periodic trends, chlorine surprisingly surpasses it due to the small size of fluorine causing repulsion among the electrons in the 2p orbital.
When studying periodic trends, always take into account the exceptions, as they often lead to a deeper understanding of atomic interactions.

Chemical Properties of Elements

The chemical properties of elements determine how they react and interact with other substances. These properties are largely influenced by the arrangement of electrons in their atoms. For halogens, being on the brink of a complete valence shell means they naturally exhibit certain chemical behaviors:

• High electron affinity - Halogens readily gain electrons.
• High reactivity - They form compounds easily, especially with alkali metals and alkaline earth metals.
• Forming diatomic molecules - In their elemental form, they typically exist as pairs, such as \( \text{F}_2 \) and \( \text{Cl}_2 \).

Their chemical properties make halogens integral components in compounds important to everyday life, from salt (sodium chloride) to various disinfectants and medications. Recognizing these properties aids in predicting reactions and compounds that halogens can form, which is crucial in fields like chemical engineering and pharmacology.