Q.In which of the following substances will hydrogen bond be strongest?

The key idea is that the strength of a hydrogen bond depends on the electronegativity of the atom bonded to hydrogen.

1. Hydrogen bonding occurs when a hydrogen atom is covalently bonded to a highly electronegative and small atom (like F, O, or N) and is attracted to another electronegative atom.
2. The strength of the hydrogen bond increases with the electronegativity of the atom to which hydrogen is directly bonded, as this makes the H-X bond more polar. …

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when a hydrogen atom is covalently bonded to a highly electronegative atom (like Fluorine, Oxygen, or Nitrogen) and is simultaneously attracted to a lone pair of electrons on another highly electronegative atom in an adjacent molecule. This interaction is crucial for many physical and biological properties of substances.

The strength of a hydrogen bond depends on several factors, but the most significant ones are:

1. Electronegativity of the atom bonded to hydrogen (X in H-X): A higher electronegativity of atom X pulls the electron density away from the hydrogen atom more effectively. This creates a larger partial positive charge ($\delta^+$) on the hydrogen atom and a larger partial negative charge ($\delta^-$) on atom X. The greater the $\delta^+$ on hydrogen, the stronger its electrostatic attraction to the lone pair of an electronegative atom in another molecule, resulting in a stronger hydrogen bond.
2. Size of the atom bonded to hydrogen (X in H-X): Smaller atoms allow for closer approach and more effective orbital overlap, which can contribute to stronger interactions. However, electronegativity is generally the dominant factor when comparing atoms within the same period or group.

Let's analyze the given options:

1. Identify the atom bonded to hydrogen in each substance.

   • (A) HCl: Hydrogen is bonded to Chlorine (Cl).
   • (B) H$_2$O: Hydrogen is bonded to Oxygen (O).
   • (C) HI: Hydrogen is bonded to Iodine (I).
   • (D) H$_2$S: Hydrogen is bonded to Sulfur (S).

2. Compare the electronegativities of Cl, O, I, and S.

   The Pauling electronegativity values for these elements are:

   • Oxygen (O): 3.44
   • Chlorine (Cl): 3.16
   • Sulfur (S): 2.58
   • Iodine (I): 2.66

   Arranging them in decreasing order of electronegativity: O > Cl > I > S.

3. Relate electronegativity to hydrogen bond strength.

   As established, a higher electronegativity of the atom bonded to hydrogen leads to a more polarized H-X bond and a greater partial positive charge on the hydrogen atom. This enhanced positive charge on hydrogen results in a stronger electrostatic attraction to the lone pair of an electronegative atom in an adjacent molecule, thereby forming a stronger hydrogen bond. …