Animated Solution for Chemistry - s and p-Block Elements: The absolute value of the electron gain enthalpy of halogens satisfies

When we dive into the periodic table, we often expect elements to follow strict, predictable rules. But nature loves to throw a curveball, and the halogens provide one of the most fascinating exceptions in chemistry. Let's explore why the electron gain enthalpy of halogens doesn't quite follow the script.

Size vs Attraction

Electron gain enthalpy, denoted as $\Delta H_{eg}$, is the energy released when a neutral gaseous atom accepts an extra electron. Because the atom reaches a more stable state, this process is generally exothermic, meaning the enthalpy value is negative.

Normally, as we travel down Group 17 from Fluorine ($F$) to Iodine ($I$), the atomic size increases. With a larger atomic radius, the incoming electron is placed further away from the positively charged nucleus. A greater distance means a weaker electrostatic attraction, which should logically result in less energy being released. Based on this, we would expect the absolute value of electron gain enthalpy to steadily decrease: $F>Cl>Br>I$.

The Crowded Room

But here is where the plot twists. Fluorine is exceptionally small. Its valence electrons reside in the highly compact $2p$ subshell. Imagine trying to squeeze a new person into a tiny, already crowded elevator. The existing passengers will push back.

Similarly, when an incoming electron tries to enter Fluorine's $2p$ orbital, it faces intense inter-electronic repulsion from the seven electrons already packed tightly inside. This repulsion counteracts the strong nuclear pull, making it harder for the new electron to settle in. As a result, less energy is released than we would theoretically expect. The value for Fluorine is $-328\text{ kJ/mol}$.

The Spacious Hall

Now, let's shift our focus to Chlorine. Chlorine is in the third period, meaning its valence electrons are in the larger $3p$ subshell. The electron density is much more diffused over a larger volume.

Returning to our analogy, Chlorine is like a spacious hall. When a new electron enters, there is plenty of room. The inter-electronic repulsion is significantly lower, allowing the nucleus to pull the electron in smoothly and efficiently. Because the electron is accommodated with less resistance, a greater amount of energy is released. Chlorine boasts an electron gain enthalpy of $-349\text{ kJ/mol}$.

Because Chlorine can accept the extra electron more easily than the highly repulsive Fluorine, its absolute electron gain enthalpy is higher. After Chlorine, the trend normalizes. Bromine and Iodine are large enough that repulsion isn't a major issue, but their large sizes mean the nuclear attraction is weaker, leading to lower energy releases ($-325\text{ kJ/mol}$ and $-295\text{ kJ/mol}$, respectively).

Therefore, the correct order for the absolute value of electron gain enthalpy is $Cl>F>Br>I$. This classic Period 2 vs Period 3 anomaly is a beautiful reminder that in chemistry, size and electron density dictate the rules of attraction!