Question

The inert gas element with the largest positive electron gain enthalpy is

• A. He
• B. Ne
• C. Kr
• D. Ar
• E. Rn

Hint:

Electron gain enthalpy becomes less negative (more positive) as we move down a group for noble gases.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Electron gain enthalpy (\(\Delta_{eg}H\)) is the enthalpy change when a neutral gaseous atom accepts an electron to form a gaseous anion. A positive electron gain enthalpy means that energy is absorbed (an endothermic process) when an electron is added, indicating that the formation of the anion is unfavorable. Inert gases have stable, completely filled valence shells (ns\(^2\)np\(^6\)), making it very difficult to add an extra electron.
Step 2: Detailed Explanation:
For an inert gas, an incoming electron must occupy a new, higher-energy principal shell (n+1). For example:

For He (1s\(^2\)), the electron enters the 2s orbital.
For Ne (2s\(^2\)2p\(^6\)), the electron enters the 3s orbital.
For Ar (3s\(^2\)3p\(^6\)), the electron enters the 4s orbital.
This process requires a significant amount of energy, resulting in positive electron gain enthalpies for all noble gases.
The question asks for the largest positive value, which means the process is the most energetically unfavorable.
The trend down the group (He to Rn) is that the atomic size increases. The incoming electron enters an orbital that is progressively farther from the nucleus and is more effectively shielded by the inner electrons. This makes the addition of an electron slightly less unfavorable as we go down the group.
Therefore, we expect the electron gain enthalpy to become less positive down the group.

He has a value of +48 kJ/mol.
Ne has a value of +116 kJ/mol.
Ar has a value of +96 kJ/mol.
Kr has a value of +96 kJ/mol.
Neon (Ne) has an anomalously high positive value. This is because the incoming electron must be added to the n=3 shell, which is relatively high in energy, and the compact size and stable 2p\(^6\) configuration of Neon strongly resists the addition of a new electron. The electron-electron repulsion is significant.
Step 3: Final Answer:
Among the given inert gases, Neon (Ne) has the largest positive electron gain enthalpy. This corresponds to option (B).