Question
Why does NH₃ act as a weaker acid but stronger base compared to PH₃, even though both are Group 15 hydrides? (JEE Main 2024 pattern)
Solution — Step by Step
Nitrogen () is more electronegative than phosphorus (). The N–H bond is more polar than the P–H bond.
The lone pair on N is held in an sp³ hybrid orbital with significant s-character — it sits in a compact, high-electron-density region. NH₃ readily donates this lone pair to a proton or Lewis acid.
PH₃, in contrast, uses nearly pure p orbitals (small bond angle, vs for NH₃). Its lone pair is more diffuse, less available for donation. Hence PH₃ is a much weaker base.
For acid behaviour, we ask: how easily does H leave as H⁺? In NH₃, the highly polar N–H bond actually holds the proton tightly because the partial negative charge on N attracts H⁺. So NH₃ is a poor proton donor.
In PH₃, the bond is less polar, but the bond is also weaker due to poorer overlap (large P, small H). Still, neither hydride is a strong acid in water; PH₃ is just slightly more acidic than NH₃.
NH₃ is a stronger base (sp³ lone pair, high s-character, compact and donor-friendly).
PH₃ is a slightly stronger acid (weaker E–H bond, less basic conjugate base).
Final answer: NH₃ wins on base strength because of its compact, sp³ lone pair; PH₃ is marginally more acidic because its E–H bond is weaker.
Why This Works
Group 15 hydride trends test two competing factors: (1) electronegativity decreasing down the group makes E–H less polar; (2) atomic size increasing down the group weakens the E–H bond. NH₃ vs PH₃ basicity is dominated by lone-pair geometry; acidity is dominated by bond strength.
This pattern repeats with H₂O vs H₂S, HF vs HCl. Going down a group, basicity decreases and acidity (in water) increases.
Alternative Method
Compare values: NH₃ (), PH₃ (). Lower means stronger acid. The order matches the bond-strength argument.
For NEET, also remember the bond angle drop: NH₃ (), PH₃ (), AsH₃ (), SbH₃ (). Down the group, p-character of bonding orbitals dominates.
Common Mistake
Saying “more electronegative N means stronger acid because the bond is more polar”. Wrong direction — high polarity means N holds H⁺ more tightly, making NH₃ a poorer proton donor. Bond polarity favours basicity, not acidity, for hydrides of small atoms.