Problem 11 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Electron gain enthalpy with negative sign of fluorine is less than that of chlorine due to : [Main Online April 9, 2013] (a) High ionization enthalpy of fluorine (b) Smaller size of chlorine atom (c) Smaller size of fluorine atom (d) Bigger size of \(2 p\) orbital of fluorine

Step-by-Step Solution

Answer

The correct option is (c) Smaller size of fluorine atom.

Step 1: Analyze the Concept

Understand that electron gain enthalpy is the energy change that occurs when an electron is added to a neutral atom. It is often negative because energy is usually released when an atom gains an electron.

Step 2: Relate to Atomic Size

Consider the size of the atoms involved. Fluorine has a smaller atomic size compared to chlorine. A smaller atom means that added electrons experience more repulsion from other electrons.

Step 3: Understand the Electron Repulsion Effect

The smaller size of fluorine leads to higher electron-electron repulsion in its small outer shell, making it less favorable to add an additional electron, even more than could be initially assumed from its small size.

Step 4: Compare with Chlorine

Chlorine, being larger, has reduced electron-electron repulsions when an additional electron is added. As a result, chlorine can accommodate an extra electron more favorably compared to fluorine.

Step 5: Select the Correct Option

Based on the above understanding, the negative electron gain enthalpy of fluorine is less than that of chlorine primarily due to the smaller atomic size of fluorine, resulting in increased electron-electron repulsion. Therefore, the correct answer is (c) Smaller size of fluorine atom.

Key Concepts

Atomic Size

Atomic size refers to the distance between the nucleus of an atom and its valence electron or outermost shell. In simple terms, it gives us an idea of how big or small an atom is. Atomic size varies within the periodic table as you move across periods or down groups.
In the context of electron gain enthalpy, the size of an atom plays a crucial role. A smaller atomic size, like that of fluorine, implies that its electrons are closer to the nucleus. This proximity can affect the atom's ability to attract additional electrons.

• Smaller atomic size means the outer electrons experience greater attraction towards the nucleus, but can also experience higher electron-electron repulsion.
• Larger atomic size, like in chlorine, means the added electron is at a lesser electron-electron repulsion due to more space in the electron cloud.

Thus, the atomic size is a fundamental factor in understanding why fluorine, despite being highly reactive, has less negative electron gain enthalpy than chlorine.

Electron-Electron Repulsion

When an electron is added to an atom, it doesn't just interact with the nucleus; it also has interactions with the rest of the electrons present. This interaction is known as electron-electron repulsion. The electrons are negatively charged and naturally repel each other.
In smaller atoms like fluorine, electrons are packed closely together. This closeness increases the repulsion among electrons and makes it difficult for the atom to accept an additional electron.

• High electron-electron repulsion reduces the tendency of an atom to gain an electron effectively.
• In larger atoms, like chlorine, this repulsion is spread over a bigger volume, reducing the impact on a new electron being added.

As a result, the electron-gaining capability, measured by electron gain enthalpy, can be surprisingly lower in smaller atoms due to this repulsion effect.

Halogens Chemistry

Halogens are elements found in Group 17 of the periodic table, including fluorine and chlorine. These elements are known for being highly reactive due to their one electron short of a full valence shell.
Halogens gain one electron readily to achieve a stable electron configuration, releasing energy in the process. However, the extent of energy released, or electron gain enthalpy, can vary among them due to their atomic and chemical properties.

• Fluorine, despite being extremely electronegative, has lower electron gain enthalpy than chlorine because of its smaller size and high electron-electron repulsion.
• Chlorine, with its larger atomic size, can accommodate the added electron more comfortably, leading to a more negative electron gain enthalpy.

Understanding the chemistry of halogens helps explain these variations in reactivity and stability among the members of this group, reflecting key periodic trends.