Problem 65 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is the correct relation with regard to the electron afinity of halogen? (a) \(\mathrm{F}>\mathrm{Cl}\) (b) \(\mathrm{F}>\mathrm{I}\) (c) \(\mathrm{Br}>\mathrm{F}\) (d) \(\mathrm{Br}>\mathrm{Cl}\)

Step-by-Step Solution

Answer

Correct relation: \( \mathrm{F} > \mathrm{I} \) (Option b)

Step 1: Understanding Electron Affinity

Electron affinity is the energy change that occurs when an electron is added to a neutral atom, and it typically increases across a period and decreases down a group in the periodic table. Halogens are known for having high electron affinities due to their desire to gain one electron to achieve a noble gas configuration.

Step 2: Analyzing Trends in Halogens

In the halogen group (group 17), the electron affinities generally decrease as you move down the group. The order based on electron affinity is usually: \[ \mathrm{Cl} > \mathrm{F} > \mathrm{Br} > \mathrm{I} \]This might seem counterintuitive because fluorine is very reactive, but chlorine actually has a higher electron affinity than fluorine due to additional electron-electron repulsions in fluorine's smaller 2p orbital.

Step 3: Evaluating Given Statements

Let's evaluate each option based on the order of electron affinity: (a) \( \mathrm{F} > \mathrm{Cl} \) - Incorrect, because \( \mathrm{Cl} \) has a higher electron affinity than \( \mathrm{F} \).(b) \( \mathrm{F} > \mathrm{I} \) - Correct, because \( \mathrm{F} \) indeed has a higher electron affinity than \( \mathrm{I} \).(c) \( \mathrm{Br} > \mathrm{F} \) - Incorrect, as \( \mathrm{F} \) has a higher electron affinity than \( \mathrm{Br} \).(d) \( \mathrm{Br} > \mathrm{Cl} \) - Incorrect, since \( \mathrm{Cl} \) has a higher electron affinity than \( \mathrm{Br} \).

Step 4: Choosing the Correct Answer

From our evaluation, only option (b) is correct, representing \( \mathrm{F} > \mathrm{I} \) in terms of electron affinity.

Key Concepts

Halogens

Halogens, which belong to Group 17 of the periodic table, are a fascinating group of elements known for their reactivity. They include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At).
What makes halogens particularly interesting is their electron configuration which typically ends in **p**-orbitals, specifically with seven electrons in their outermost energy level. This configuration makes them one of the most reactive groups, needing just one additional electron to achieve a noble gas configuration.

• Their high reactivity is because they readily gain electrons.
• Halogens have some of the highest electronegativities among elements.
• They typically have high electron affinities due to their strong attraction for extra electrons.

A common property among halogens is their ability to form diatomic molecules, like \(F_2\) or \(Cl_2\), enhancing their stability. When halogens gain an extra electron, they form \(X^-\) ions, which are stable and resemble the electron configuration of noble gases.

Periodic Trends

Understanding periodic trends such as electron affinity can provide insight into the reactivity and properties of elements across the periodic table. As you move across a period from left to right, electron affinity generally increases. This is due to increasing nuclear charge that attracts electrons more strongly.
However, when moving down a group, electron affinity generally decreases as larger atomic size leads to greater distance between the nucleus and the added electron, reducing the attraction force.

• In the halogen group, chlorine exhibits an unusual higher electron affinity than fluorine despite trends.
• Down the group, electron affinity decreases: \(\mathrm{Cl} > \mathrm{F} > \mathrm{Br} > \mathrm{I}\).
• This illustrates that trends can have exceptions due to other influencing factors like electron-electron repulsion.

These periodic trends provide a framework for predicting how elements will react.

Noble Gas Configuration

Achieving a noble gas configuration is often the goal for many elements due to its stability. Noble gases, found in Group 18, are characterized by their full valence shells (completely filled _s_ and _p_ orbitals). This makes them inert and largely unreactive as they have no tendency to gain or lose electrons.
Halogens, with their seven valence electrons, strive to attain this stable configuration by gaining an electron. This action transforms them into fully stable ions with electron configurations identical to the nearest noble gas.

• For instance, when fluorine gains an electron, it achieves the electron configuration of neon, a noble gas.
• Chlorine gaining an electron will result in an electron configuration resembling argon.
• These changes are responsible for the high electron affinity observed in halogens.

The tendency to achieve a noble gas configuration helps explain the behavior of many elements, as attaining stability is key.