Question

Strongest hydrogen bond is present in

• A. $\mathrm{H}-\mathrm{O}\dots \dots \dots \mathrm{H}-\mathrm{O}$
• B. $\mathrm{H}-\mathrm{N}\dots \dots \dots \mathrm{H}-\mathrm{N}$
• C. $\mathrm{H}-\mathrm{F}\dots \dots \dots \mathrm{H}-\mathrm{F}$
• D. $\mathrm{H}-\mathrm{Cl}\dots \dots \dots \mathrm{H}-\mathrm{Cl}$

Answer 1

The correct option is C

$\mathrm{H}-\mathrm{F}\dots \dots \dots \mathrm{H}-\mathrm{F}$

Explanation for correct option:

(C) $\mathrm{H}-\mathrm{F}\dots \dots \dots \mathrm{H}-\mathrm{F}$

Hydrogen Bond:

• A Hydrogen bond (or $\mathrm{H}$-bond) is an electrostatic attraction force.
• The attractive force between a hydrogen atom covalently bound to a quite electronegative atom such as an $\mathrm{N},\mathrm{O}\mathrm{or}\mathrm{F}$ atom and another extremely electronegative atom causes it.
• Hydrogen bonding is not a covalent link to a hydrogen atom, but rather a sort of dipole-dipole attraction between molecules.
• The Electronegativity of the hydrogen atom with other atoms is shown in the table.

Element	Electronegativity Value
$\mathrm{H}$	$2.1$
$\mathrm{N}$	$3.0$
$\mathrm{O}$	$3.5$
$\mathrm{F}$	$4.1$
$\mathrm{Cl}$	$3.2$

.

• Therefore, Fluorine is the most electronegative atom. As a result, it can form the strongest hydrogen bond.

Explanation for incorrect option:

(A) $\mathrm{H}-\mathrm{O}\dots \dots \dots \mathrm{H}-\mathrm{O}$

• Oxygen possesses an unshared electron pair, giving it a minor negative charge resulting in a slight positive charge over the Hydrogen atom.
• The electronegativity of oxygen with hydrogen is not as high as fluorine.

(B) $\mathrm{H}-\mathrm{N}\dots \dots \dots \mathrm{H}-\mathrm{N}$

• Because the charge on the $\mathrm{F}$ nucleus is +9, while the charge on the $\mathrm{N}$ nucleus is $+7$, fluorine is more electronegative than nitrogen.
• Therefore its hydrogen bond is not as strong as Fluorine.

(D) $\mathrm{H}-\mathrm{Cl}\dots \dots \dots \mathrm{H}-\mathrm{Cl}$

• The Electronegativity value of chlorine is less than the Fluorine atom.
• Therefore it has a weak hydrogen bond than Fluorine.

Hence option (C) is correct