Considering electronegativity differences, which hydrogen bond among the following has the greatest ionic character?

Introduction / Context:
This question examines your understanding of bond polarity and ionic character based on electronegativity differences between bonded atoms. Covalent bonds can vary in how evenly electrons are shared. When the electronegativity difference between two atoms is large, the bond becomes strongly polar and may be described as having significant ionic character. Here, you must identify which hydrogen bond, H–X, has the greatest ionic character by comparing electronegativities of the second element in each pair.

Given Data / Assumptions:

• The bonds compared are H–F, H–Cl, H–N, H–O and H–S.
• Hydrogen has moderate electronegativity, roughly around 2.1 on the Pauling scale.
• Fluorine is the most electronegative element, followed by oxygen, chlorine, nitrogen and sulfur among the listed atoms.
• Greater electronegativity difference implies stronger bond polarity and more ionic character.

Concept / Approach:
Bond ionic character is an approximate concept that increases with the difference in electronegativity between the two atoms in a bond. The larger the difference, the more the electrons are drawn toward the more electronegative atom, making the bond more polar and closer in behaviour to an ionic bond. On the Pauling scale, fluorine has the highest electronegativity of about 4.0, while hydrogen is about 2.1, giving a difference near 1.9. Oxygen, chlorine and nitrogen have lower electronegativities than fluorine, so their differences with hydrogen are smaller. Therefore, among the listed bonds, H–F has the largest electronegativity difference and thus the greatest ionic character.

Step-by-Step Solution:
Step 1: Recall approximate electronegativities: H ~ 2.1, F ~ 4.0, O ~ 3.5, Cl ~ 3.0, N ~ 3.0, S ~ 2.5. Step 2: Compute rough differences with hydrogen: H–F ~ 1.9, H–O ~ 1.4, H–Cl ~ 0.9, H–N ~ 0.9, H–S ~ 0.4. Step 3: Recognise that the largest difference among these is for the H–F bond. Step 4: A greater electronegativity difference means the electron pair is more strongly attracted to the more electronegative atom, creating a larger partial charge separation. Step 5: This greater charge separation corresponds to higher bond polarity and greater ionic character. Step 6: Therefore, among the given options, H–F has the greatest ionic character.

Verification / Alternative check:
You can verify this by looking at properties like bond dipole moments and acid strength. Hydrogen fluoride has a very strong bond dipole because fluorine draws electron density extremely strongly. In water, HF behaves as a weak acid compared with HCl, but this is due to bond strength and solvation, not lack of polarity. Comparing other hydrogen compounds, such as H2O, HCl and NH3, shows that their bonds are polar but none exhibit as extreme a charge separation as H–F. This is also reflected in the fact that fluorine forms the most polar covalent bonds of all elements, reinforcing that H–F has high ionic character among the options.

Why Other Options Are Wrong:
H–Cl has a significant electronegativity difference, but chlorine is less electronegative than fluorine, so H–Cl is less polar than H–F. H–N and H–O are also polar, but the electronegativity differences are smaller than that for H–F, giving them less ionic character. H–S has the smallest difference among the options; sulfur is only slightly more electronegative than hydrogen, so the bond is comparatively less polar. Therefore, none of these bonds surpass H–F in ionic character.

Common Pitfalls:
Students sometimes misremember electronegativity order, thinking oxygen might be the most electronegative element rather than fluorine. This can lead them to incorrectly choose H–O. To avoid this, remember the simple rule that fluorine is always the most electronegative element in the periodic table. Another mistake is to confuse bond strength with ionic character; while H–F has a strong bond, the question is about polarity, not necessarily about how easily the bond breaks. Always focus on electronegativity differences when comparing ionic character of covalent bonds.

Final Answer:
The hydrogen bond with the greatest ionic character among the options is H–F.