Problem 82 - A Complete Resource Book in Chemistry for JEE Main

Question

The electronic affinity values (in \(\mathrm{kJ} \mathrm{mol}^{-1}\) ) of three halogens \(\mathrm{X}, \mathrm{Y}\) and \(\mathrm{Z}\) are respectively \(-349,-333\) and \(-325\). Then \(\mathrm{X}, \mathrm{Y}\) and \(\mathrm{Z}\) respectively are (a) \(\mathrm{F}_{2}, \mathrm{Cl}_{2}\) and \(\mathrm{Br}_{2}\) (b) \(\mathrm{Cl}_{2}, \mathrm{~F}_{2}\) and \(\mathrm{Br}_{2}\) (c) \(\mathrm{Cl}_{2}, \mathrm{Br}_{2}\) and \(\mathrm{F}_{2}\) (d) \(\mathrm{Br}_{2}, \mathrm{Cl}_{2}\) and \(\mathrm{F}_{2}\)

Step-by-Step Solution

Answer

The halogens are Cl₂, F₂, and Br₂ corresponding to option (b).

Step 1: Understand Electron Affinity

Electron affinity is the energy released when an electron is added to a gaseous atom. Higher negative values indicate stronger attraction to electrons.

Step 2: Arrange Halogens by Electron Affinity

The given electron affinities are \(-349\), \(-333\), and \(-325\). Arrange these values in order of increasing negative electron affinity: \(-325 > -333 > -349\).

Step 3: Match Halogens with Electron Affinity

Fluorine \((\mathrm{F}_2)\) typically has the highest electron affinity among halogens, followed by Chlorine \((\mathrm{Cl}_2)\), and then Bromine \((\mathrm{Br}_2)\). The most negative affinity \(-349\) matches Chlorine, \(-333\) matches Fluorine, and \(-325\) matches Bromine.

Step 4: Identify Correct Option

With the arrangement: \(-349 = \mathrm{Cl}_2\), \(-333 = \mathrm{F}_2\), \(-325 = \mathrm{Br}_2\), the order \( \mathrm{X}, \mathrm{Y}, \mathrm{Z} \) matches option (b): \(\mathrm{Cl}_2, \mathrm{F}_2, \mathrm{Br}_2\).

Key Concepts

Halogens

Halogens are fascinating elements that belong to Group 17 of the periodic table. They include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These elements are known for being highly reactive, especially when it comes to gaining electrons to form anions.
Their reactivity can be understood from their position on the periodic table:

• They have seven electrons in their outermost shell.
• They need just one more electron to achieve the stable electron configuration similar to noble gases.

This is why they are keen on forming negative ions (anions) by gaining an electron, which also plays into their high electron affinities. Fluorine is the most reactive, making it quite a star among the halogens, followed closely by chlorine and then bromine.

Chemical Properties

The chemical properties of halogens are largely defined by their tendency to gain electrons. This makes them highly effective oxidizing agents, which means they easily take electrons from other substances.
There are a few key points to remember about their chemical behavior:

• They form salts when combined with metals (e.g., NaCl - table salt).
• They have strong electronegativity, with fluorine being the most electronegative element.
• The reactivity of halogens decreases down the group from fluorine to iodine.
• Halogens tend to form diatomic molecules (e.g., Cl₂, Br₂), signifying their need to gain stability through electron sharing.

These properties explain their practical applications such as water purification (chlorine) and as antiseptics (iodine).

Gaseous Atoms

When we talk about gaseous atoms, we are referring to individual atoms in the gaseous phase. For halogens, they don't exist naturally as single atoms because they are more stable as diatomic molecules. That's why we often see them as F₂, Cl₂, etc., in their gaseous state.
This is essential to understanding electron affinity, as electron affinity values measure how much energy is released when an electron is added to these gaseous atoms.

• In the gaseous state, atoms are isolated, making the electron affinity measures particularly useful in evaluating how much these atoms want to gain an electron.
• This isolated condition is what allows them to show their true "desire" for additional electrons, hence their higher electron affinities compared to other states of matter.

Knowing about gaseous atoms can enhance our understanding of atomic interactions in various chemical processes.