Q.Among halogens, the correct order of amount of energy released in electron gain (electron gain enthalpy) is:

The key concept here is the electron gain enthalpy trend within a group, specifically the anomaly observed for fluorine.

1. Generally, as we move down a group, the atomic size increases, and the effective nuclear charge experienced by the incoming electron decreases. This leads to a weaker attraction for the incoming electron, making the electron gain enthalpy less negative (less energy is released).
2. Halogens have a strong tendency to gain an electron, resulting in highly negative electron gain enthalpies.
3. However, fluorine (F) is an exception. Due to its extremely small atomic size, the incoming electron experiences significant interelectronic repulsion from the already present electrons in the compact 2p subshell. …

Electron gain enthalpy ($\Delta_{eg}H$) is the energy change that occurs when an electron is added to a neutral gaseous atom to form a negative ion. By convention, if energy is released during this process, $\Delta_{eg}H$ is negative. A more negative value indicates a greater tendency of the atom to accept an electron, meaning a larger amount of energy is released. The question asks for the "amount of energy released," which refers to the magnitude of the electron gain enthalpy, i.e., $|\Delta_{eg}H|$. A higher value of $|\Delta_{eg}H|$ means a greater amount of energy released.

Let's analyze the trend for halogens (F, Cl, Br, I):

1. General Trend Down a Group:

   As we move down a group in the periodic table, the atomic size increases. This means the valence shell is further away from the nucleus, and the nuclear attraction for an incoming electron decreases due to increased shielding by inner electrons. Consequently, the tendency to accept an electron decreases, and less energy is released.

   Based on this general trend, we would expect the order of the amount of energy released to be F > Cl > Br > I. However, fluorine is an exception.

2. The Anomaly of Fluorine:

   Fluorine is the smallest element in the halogen group. Its valence shell (2p subshell) is very compact. When an incoming electron attempts to enter this already small and electron-dense 2p subshell, it experiences significant interelectronic repulsion from the existing electrons. This strong repulsion makes the addition of an electron less favorable than expected.

   Chlorine, on the other hand, is larger than fluorine. Its valence electrons are in the 3p subshell, which is more diffuse. The interelectronic repulsion experienced by an incoming electron in chlorine's larger valence shell is much less significant compared to fluorine. As a result, chlorine can accommodate an additional electron more easily and releases a greater amount of energy than fluorine.

   Watch out

   A common misconception is to assume that because fluorine is the most electronegative element, it must also have the most negative electron gain enthalpy. While electronegativity relates to the attraction for shared electrons in a bond, electron gain enthalpy relates to the energy change when an isolated atom gains an electron. The small size and high electron density of fluorine lead to significant interelectronic repulsion, which makes its electron gain enthalpy less negative than that of chlorine.

3. Combining the Trends:

   • Due to the anomaly, chlorine has a greater amount of energy released than fluorine: $|\Delta_{eg}H_{Cl}| > |\Delta_{eg}H_F|$. …