Ammonium Ion as a Weak Acid

Steps:

• Recall NH4+ structure: tetrahedral geometry with four equivalent N-H bonds from sp3 hybridization.
• Apply VSEPR theory: bond angles are 109.5°, not 90°.
• Assess acid-base behavior: NH4+ donates H+ to form NH3, confirming weak acid properties.
• Eliminate options: A and B misstate geometry; C confuses bonding with neutral ammonia.

Why D is correct:

• NH4+ acts as a weak acid per Brønsted-Lowry definition, dissociating as NH4+ ⇌ NH3 + H+ (Ka = 5.6 × 10^-10), reacting with bases like OH- to form NH3 + H2O.

Why the others are wrong:

• A: H-N-H angles are 109.5° in tetrahedral structure, not 90°.
• B: N-H bonds are lengths, not angles; this misinterprets molecular geometry.
• C: All four N-H bonds in NH4+ are identical covalent bonds, with no weaker dative bond.

Final answer: D