Question
You’re working in a fertilizer plant. Why is ammonia () a base while phosphine () is a much weaker base, despite both being Group 15 hydrides? And why does form hydrogen bonds while doesn’t, affecting their boiling points dramatically?
Solution — Step by Step
Both NH₃ and PH₃ have a lone pair on the central atom. Basicity depends on how readily this lone pair is donated.
In NH₃, nitrogen is small and electronegative — the lone pair is concentrated and “sticks out” sharply. Highly available for protonation.
In PH₃, phosphorus is larger; the lone pair is more diffuse and held in a larger orbital. Less concentrated, less available for donation. So NH₃ is the stronger base.
NH₃ uses sp³ hybridisation — the lone pair sits in an sp³ orbital, partially directional.
PH₃ uses pure p orbitals for the three P-H bonds; the lone pair sits in an s orbital. This s-character makes the lone pair less directional, harder to donate.
Hydrogen bonding requires H attached to a small, highly electronegative atom (F, O, N) and another such atom nearby. NH₃ qualifies; PH₃ doesn’t (phosphorus is not electronegative enough).
Boiling point of NH₃: -33°C. Boiling point of PH₃: -88°C. The 55°C difference comes almost entirely from H-bonding in NH₃.
NH₃ stronger base + H-bonded; PH₃ weaker base + no H-bonding.
Why This Works
Group 15 hydrides illustrate a general trend: as you go down the group, atomic size increases, electronegativity decreases, and the central atom becomes a poorer base and weaker H-bond acceptor.
Boiling point trend in Group 15 hydrides: . The dip at PH₃ is due to NH₃’s anomalous H-bonding boost.
Whenever you see boiling point anomalies in hydrides (HF vs HCl, H₂O vs H₂S, NH₃ vs PH₃), suspect hydrogen bonding. The lighter hydride is anomalously high in BP because of H-bonds.
Alternative Method
Energetic comparison: pKa of NH₄⁺ ≈ 9.2, pKa of PH₄⁺ ≈ -14. So PH₄⁺ is a much stronger acid, meaning PH₃ is a much weaker base. Same conclusion via thermodynamics.
Common Mistake
Students often think “larger atom, more lone-pair density” — wrong. Larger atom = more diffuse lone pair = lower density at any point = harder to donate. Atom size and basicity often go inversely in a group.