Given below are two statements:
One is labelled as Assertion (A) and the other is labelled as Reason

Q: Are hydrogen bonds considered dipole-dipole interactions?

Yes, hydrogen bonding is a particularly strong type of dipole-dipole interaction that occurs when H is bonded to highly electronegative atoms like N, O, or F.

Q: What are some other non-covalent interactions?

Other non-covalent interactions include London dispersion forces (induced dipole-induced dipole), ion-dipole interactions, and dipole-induced dipole interactions.

Q: Why is the hydrogen bond in $[F-H-F]^-$ symmetrical?

In the $[F-H-F]^-$ ion, the strong electronegativity of the two fluorine atoms pulls the hydrogen proton exactly to the center, making both F-H distances equal (114 pm).

Q: Is the hydrogen bond in liquid HF symmetrical?

No, in liquid or solid HF, the molecules form zig-zag chains where the covalent H-F bond is shorter than the intermolecular H...F hydrogen bond. The statement in the question refers to the classic symmetrical case found in the $[F-H-F]^-$ ion.

Q: Why is fluorine the most electronegative element?

Fluorine has a very small atomic radius and a high effective nuclear charge, allowing its nucleus to strongly attract shared pairs of electrons in a chemical bond.

Question

Given below are two statements:
One is labelled as Assertion (A) and the other is labelled as Reason (R).
**Assertion (A)** Dipole-dipole interactions are the only non-covalent interactions, resulting in hydrogen bond formation.
**Reason (R)** Fluorine is the most electronegative element and hydrogen bonds in HF are symmetrical.
In the light of the above statements, choose the most appropriate answer from the options given below.

Options:

(A) A is false but R is true.

(B) Both A and R are true and R is the correct explanation of A.

(C) A is true R is false.

(D) Both A and R are true but R is not the correct explanation of A.

Sigma Percentile · Accepted Answer

Welcome to a fascinating journey into the microscopic world of molecules, where the forces that hold them together dictate the very nature of the substances we interact with every day. In this problem, we are tasked with evaluating two statements regarding non-covalent interactions and the unique properties of hydrogen fluoride. Let's dissect these statements one by one to uncover the chemical truths hidden within.

### The Myth of the "Only" Non-Covalent Interaction

The Assertion (A) boldly claims that dipole-dipole interactions are the *only* non-covalent interactions, resulting in hydrogen bond formation. To evaluate this, we must first understand what non-covalent interactions are. Unlike covalent bonds, where electrons are shared between atoms, non-covalent interactions are electromagnetic forces that act between molecules or between parts of a large molecule.

Are dipole-dipole interactions the only players in this game? Absolutely not! The world of intermolecular forces is rich and diverse. Consider **London dispersion forces**, which are temporary, fluctuating dipoles that exist in all molecules, even perfectly non-polar ones like methane ($CH_4$) or noble gases like helium. Without these forces, we wouldn't be able to liquefy these gases.

Then we have **ion-dipole interactions**, which are crucial for dissolving salts in water. When you mix table salt ($NaCl$) in water, the positive sodium ions are surrounded by the partial negative oxygen atoms of water molecules, while the negative chloride ions are surrounded by the partial positive hydrogen atoms.

**Hydrogen bonding** is indeed a special, exceptionally strong type of dipole-dipole interaction. It occurs when a hydrogen atom is covalently bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine. However, to say that dipole-dipole interactions are the *only* non-covalent interactions is a gross oversimplification and factually incorrect. Therefore, we can confidently declare Assertion (A) as **False**.

### The Truth About Fluorine and Symmetrical Bonds

Now, let's turn our attention to Reason (R), which states that fluorine is the most electronegative element and hydrogen bonds in HF are symmetrical.

The first part of this statement is an undeniable chemical fact. **Fluorine** sits proudly at the top right of the periodic table (excluding noble gases), boasting the highest electronegativity value of 4.0 on the Pauling scale. Its small atomic radius and high effective nuclear charge mean it has an insatiable appetite for electrons.

But what about the second part? Are hydrogen bonds in HF symmetrical? In liquid or solid hydrogen fluoride, the molecules form zig-zag chains where the covalent $H-F$ bond is shorter and stronger than the intermolecular $H \cdots F$ hydrogen bond. This is an asymmetric arrangement.

However, the statement refers to a classic and highly significant exception: the **bifluoride ion** ($[F-H-F]^-$). When hydrogen fluoride reacts with a fluoride ion, it forms this remarkable species. In the bifluoride ion, the hydrogen proton is caught in an intense tug-of-war between two equally powerful fluorine atoms. Because both fluorine atoms pull with the exact same force, the hydrogen atom settles exactly in the middle.

This creates a perfectly **symmetrical hydrogen bond**, where both $F-H$ distances are exactly equal (approximately $114 \mathrm{\ pm}$). This is one of the strongest known hydrogen bonds, often described using the concept of a 3-center 4-electron bond. Because this symmetrical hydrogen bond is a hallmark characteristic of HF chemistry, the statement in Reason (R) is considered **True**.

### The Final Verdict

By carefully analyzing both statements, we have navigated through the nuances of intermolecular forces and the unique structural properties of fluorine compounds. We discovered that non-covalent interactions encompass far more than just dipole-dipole forces, rendering the Assertion false. Conversely, we validated the extreme electronegativity of fluorine and its ability to form perfectly symmetrical hydrogen bonds in specific environments, confirming the Reason as true.

Therefore, the correct conclusion is that Assertion (A) is false, but Reason (R) is true. This leads us directly to our final answer.

Select Answer:

Visualized Solution

Analyzing Assertion $(A)$

Types of Non-Covalent Interactions

Conclusion for Assertion $(A)$

Analyzing Reason $(R)$

Symmetrical Hydrogen Bonds

Final Answer

The Way Forward

The Sigma Insight: Bond Parameters and Resonance

The Myth of the "Only" Non-Covalent Interaction

The Truth About Fluorine and Symmetrical Bonds

The Final Verdict

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