The correct order of electron gain enthalpy with negative sign of F, Cl, Br and I, having atomic num

Q: Why is electron gain enthalpy negative?

When an atom accepts an electron, it generally achieves a more stable state, releasing energy. In thermodynamics, energy released is denoted with a negative sign.

Q: Why does electron gain enthalpy become less negative down the group?

As atomic size increases down the group, the distance between the nucleus and the incoming electron increases. This reduces the effective nuclear attraction, so less energy is released.

Q: If Fluorine is the most electronegative element, shouldn't it have the highest electron gain enthalpy?

Electronegativity is the tendency to attract shared electrons in a bond, while electron gain enthalpy is the energy released when an isolated atom gains an electron. Fluorine's extremely small size causes high repulsion for a completely new incoming electron, making its electron gain enthalpy less negative than Chlorine's.

Q: Does this anomaly happen in other groups too?

Yes! A similar anomaly occurs in Group 16, where Oxygen has a less negative electron gain enthalpy than Sulfur due to the same small size and high inter-electronic repulsion.

Q: What is the difference between electron affinity and electron gain enthalpy?

They are closely related. Electron affinity is defined at absolute zero and is the negative of the enthalpy change. Practically, they follow the same trends, but electron gain enthalpy is the rigorous thermodynamic term.

Question

The correct order of electron gain enthalpy with negative sign of F, Cl, Br and I, having atomic number 9, 17, 35 and 53 respectively, is

Options:

(A) I > Br > Cl > F

(B) F > Cl > Br > I

(C) Cl > F > Br > I

(D) Br > Cl > I > F

Sigma Percentile · Accepted Answer

### The Concept of Electron Gain Enthalpy

Imagine an isolated gaseous atom floating in space. When an extra electron is added to this atom, it transitions into a more stable anion, releasing energy in the process. This released energy is known as **electron gain enthalpy** ($\Delta_{eg}H$).

Because energy is released, the sign of this enthalpy change is negative. The more an atom 'wants' an electron, the more energy it releases, resulting in a highly negative electron gain enthalpy.

### The Expected Trend Down the Group

As we travel down Group 17 (the halogens) from Fluorine ($	ext{F}$) to Iodine ($	ext{I}$), the atomic size steadily increases. With each step down, a new electron shell is added.

Because the atom gets larger, the positively charged nucleus is further away from the outermost shell. This means the incoming electron feels a weaker attractive pull from the nucleus.

Naturally, we would expect the energy released to decrease as we go down the group. The expected order of electron gain enthalpy (with a negative sign) would logically be: $	ext{F} > 	ext{Cl} > 	ext{Br} > 	ext{I}$.

### The Fluorine Anomaly

But nature loves exceptions! Let's zoom in on the Fluorine atom. Fluorine is exceptionally small. Its outermost shell is the $2p$ subshell, which is very compact.

Fluorine already has 7 valence electrons crowded into this tiny space. When a new electron tries to enter, it faces massive **inter-electronic repulsion** from the existing electrons.

This strong repulsion acts like a shield, making it harder for the new electron to settle in. As a result, the energy released is significantly less than we would expect.

### Chlorine Takes the Crown

Now look at Chlorine. It is larger than Fluorine, with its outermost electrons in the more spacious $3p$ subshell. The incoming electron doesn't face the same intense crowding and repulsion.

Because Chlorine has a high effective nuclear charge but enough space to comfortably accommodate the extra electron, it releases the maximum amount of energy.

In fact, Chlorine has the highest negative electron gain enthalpy in the entire periodic table!

### The Final Verdict

After Chlorine, the normal trend resumes. Bromine is larger than Chlorine, so it releases less energy, and Iodine releases even less.

Therefore, the actual correct order of electron gain enthalpy (with a negative sign) is: **$	ext{Cl} > 	ext{F} > 	ext{Br} > 	ext{I}$**.

This is a classic, high-yield concept for JEE. Always remember: while Fluorine is the king of electronegativity, Chlorine wears the crown for electron gain enthalpy!

Animated Solution for Chemistry - Periodicity in Properties: The correct order of electron gain enthalpy with negative sign of F, Cl, Br and I, having atomic number 9, 17, 35 and 53 respectively, is

Select Answer:

Visualized Solution

$Electron Gain Enthalpy$

$General Trend Down the Group$

$The Fluorine Anomaly$

$Final Correct Order$

The Sigma Insight: Periodic Table and Periodic Properties

The Concept of Electron Gain Enthalpy

The Expected Trend Down the Group

The Fluorine Anomaly

Chlorine Takes the Crown

The Final Verdict

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