PH3 Molecular Polarity (Bond Angle Guide)
PH₃, or phosphine, is trigonal pyramidal and weakly polar. Its H–P–H bond angle is about 93.5°, not 109.5°. Phosphorus has three P–H bonds and one lone pair. Although each P–H bond is nearly nonpolar, their dipoles do not cancel fully in the pyramid, leaving PH₃ with a small molecular dipole of about 0.58 D.
A common source of confusion is that two familiar facts seem to point in different directions: PH₃ has a lone pair, yet its bond angle is far below the angle many students expect for a tetrahedral arrangement. Its P–H bonds are also nearly nonpolar, but the molecule itself is not nonpolar.
The solution is to separate three ideas: the arrangement of electron domains, the measured angle between bonds, and the net dipole of the whole molecule. I often see these ideas blended together in chemistry questions. Once you keep them distinct, you can explain PH₃’s shape and polarity without relying on a shortcut that gives the wrong answer.
PH₃ Structure and Molecular Polarity: What Determines the Answer?
PH₃ is phosphine, a molecule made of one phosphorus atom and three hydrogen atoms. Its shape comes from three P–H bonds and one lone pair on phosphorus. Its polarity depends on the small bond dipoles and the way those dipoles add in the molecule’s three-dimensional shape.
Start with the Lewis structure
A Lewis structure shows how valence electrons are arranged as bonds and lone pairs. Phosphorus has five valence electrons, and each of the three hydrogen atoms has one. That gives PH₃ a total of eight valence electrons.
Three pairs of electrons form three single P–H bonds. The remaining pair stays on phosphorus as a lone pair:
- Three P–H bonds use six electrons.
- One lone pair on phosphorus uses two electrons.
- Total: eight valence electrons.
This count helps explain both the shape and the polarity. The three bonds connect phosphorus to hydrogen, while the lone pair takes up space around the central atom without forming another bond.
For a quick check, phosphorus is the central atom, each hydrogen forms one bond, and no electrons remain unassigned after the lone pair is placed on phosphorus.
Lewis Structure, VSEPR Geometry, and the H–P–H Bond Angle
VSEPR is a model that predicts shape by treating electron groups around a central atom as areas that repel one another. PH₃ has four such groups: three bonds and one lone pair. Their arrangement and the molecule’s measured bond angle are related, but they are not the same thing.
Distinguish electron geometry from molecular geometry
In VSEPR notation, PH₃ is AX₃E. “A” is the central phosphorus atom, “X₃” represents its three bonded hydrogen atoms, and “E” represents its lone pair. The four electron groups have a tetrahedral electron-domain geometry.
Molecular geometry describes the positions of the atoms only. Since the lone pair is not an atom, the positions of the three hydrogen atoms make a trigonal-pyramidal shape. Phosphorus sits above the triangular base formed by the hydrogens.
| Feature | PH₃ description | What it tells you |
|---|---|---|
| Valence electrons | 8 total | 5 from P, 3 from H |
| VSEPR type | AX₃E | 3 bonds and 1 lone pair |
| Electron-domain geometry | Tetrahedral | Four electron groups around P |
| Molecular geometry | Trigonal pyramidal | Three H atoms form a pyramid |
| H–P–H angle | About 93.5° | Measured angle between two P–H bonds |
Why is the bond angle about 93.5°?
The H–P–H angle in PH₃ is about 93.5°. This is the observed bond angle, not the ideal tetrahedral angle of 109.5°. A tetrahedral arrangement is a useful starting point in VSEPR, but it does not mean every molecule with four electron groups has bond angles of exactly 109.5°.
That distinction matters in homework and exams. The four electron groups give a tetrahedral electron-domain arrangement in the basic VSEPR model, while the actual angle depends on the molecule’s electronic structure. In PH₃, the P–H bonds have substantial p-orbital character. A simple introductory explanation is that they point closer to perpendicular directions, leading to an angle near 90°.
This model helps explain the small angle, but it is not a full calculation of the molecule’s structure. Use approximately 93.5° when asked for PH₃’s H–P–H bond angle. Do not substitute 109.5° just because the molecule has four electron groups.
How to Analyze PH₃ Bond Dipoles and Molecular Dipole
A bond dipole is a small separation of charge across a bond. A molecular dipole is the overall separation of charge in the whole molecule. PH₃ has nearly nonpolar P–H bonds, but its bond dipoles do not cancel completely, so the molecule has a small, nonzero dipole.
Compare the electronegativities
Electronegativity describes how strongly an atom attracts shared electrons in a bond. On the Pauling scale, phosphorus is about 2.19 and hydrogen about 2.20. Because these values are almost equal, a P–H bond has very little polarity.
The difference is so small that simplified diagrams may show little or no bond-dipole arrow. Still, “nearly nonpolar bonds” does not automatically mean “nonpolar molecule.” To decide whether a whole molecule is polar, consider both the bond dipoles and the three-dimensional arrangement of the bonds.
Add the bond dipoles as vectors
A dipole is a vector: it has both size and direction. In a flat, symmetric arrangement, equal bond dipoles can cancel. In PH₃, the three P–H bonds point in different directions in a trigonal-pyramidal shape, so their small dipoles do not cancel perfectly.
The resulting molecular dipole is about 0.58 debye (D), a unit used to measure dipole moments. This value is small, but it is not zero. For introductory chemistry questions, the appropriate conclusion is that PH₃ is weakly polar.
It is useful to remember that the molecular dipole describes the molecule’s total charge distribution. It is not always possible to find its exact value by simply adding textbook bond-dipole arrows. The measured dipole reflects the molecule’s full electronic structure.
A reliable reasoning sequence is:
- Draw the Lewis structure and count the valence electrons.
- Identify the three bonds and the lone pair on phosphorus.
- Use AX₃E to predict trigonal-pyramidal molecular geometry.
- State the H–P–H angle as about 93.5°.
- Compare P and H electronegativities, then consider how the dipoles add.
- Conclude that PH₃ is weakly polar, with a dipole of about 0.58 D.
Common Misconceptions and Applications in Chemical Reasoning
Common mistakes include using 109.5° as PH₃’s measured bond angle and calling the molecule nonpolar because P and H have nearly equal electronegativities. Check the geometry and the whole-molecule dipole before choosing either answer.
Compare the common claims with the better conclusion
| Claim | Why it can seem reasonable | Better conclusion |
|---|---|---|
| “PH₃ is tetrahedral.” | Four electron groups surround phosphorus. | The electron-domain geometry is tetrahedral; the molecular geometry is trigonal pyramidal. |
| “Its angle is 109.5°.” | That is the ideal tetrahedral angle. | PH₃’s H–P–H angle is about 93.5°. |
| “P–H bonds are almost nonpolar, so PH₃ is nonpolar.” | P and H have nearly equal electronegativities. | PH₃ is weakly polar because its small dipoles do not cancel fully. |
| “A lone pair always makes the bond angle close to 109.5°.” | VSEPR uses tetrahedral electron domains as a starting model. | The measured angle depends on the molecule’s electronic structure. |
A short worked reasoning example
Suppose a question asks for PH₃’s shape, bond angle, and polarity. First count eight valence electrons, then draw three P–H bonds and one lone pair on phosphorus. That gives AX₃E: tetrahedral electron-domain geometry and trigonal-pyramidal molecular geometry.
Next, report the measured H–P–H angle, about 93.5°. Finally, compare electronegativities: P and H are nearly equal, so the bonds are only slightly polar. Their dipoles do not cancel completely in the pyramidal molecule, so PH₃ is weakly polar. This explanation avoids treating one fact, such as the small electronegativity difference, as the full answer.
Conclusion: the three facts to keep together
PH₃ has a trigonal-pyramidal molecular shape, an H–P–H angle of about 93.5°, and a small but nonzero molecular dipole of about 0.58 D. The 109.5° value is the ideal tetrahedral angle, not PH₃’s measured bond angle. Nearly nonpolar bonds can still form a weakly polar molecule when their dipoles do not cancel.
Frequently Asked Questions
These short answers review the key terms and figures used to describe PH₃. Read them as a final check: the shape, angle, and polarity are connected, but each answers a different chemistry question.
Is PH₃ polar or nonpolar?
PH₃ is weakly polar, not strictly nonpolar. Its P–H bonds are nearly nonpolar because phosphorus and hydrogen have similar electronegativities. However, the molecule’s trigonal-pyramidal shape means the small bond dipoles do not cancel completely. Its molecular dipole is about 0.58 D.
What is the bond angle in PH₃?
The H–P–H bond angle in phosphine is about 93.5°. This is the angle between two P–H bonds. It is not the ideal tetrahedral angle of 109.5°, which is a useful reference for electron-domain geometry rather than PH₃’s measured bond angle.
What is the molecular geometry of PH₃?
PH₃ has a trigonal-pyramidal molecular geometry. Phosphorus is the central atom, with three hydrogen atoms bonded to it and one lone pair. The lone pair is included when describing the electron-domain geometry, but it is not an atom in the molecular shape.
Why is PH₃ called AX₃E?
AX₃E is VSEPR notation for a central atom, A, bonded to three surrounding atoms, X₃, with one lone pair, E. In PH₃, phosphorus is A, the three hydrogens are X₃, and the lone pair on phosphorus is E.
How many valence electrons does PH₃ have?
PH₃ has eight valence electrons in total. Phosphorus contributes five, and the three hydrogen atoms contribute one each. Six electrons form three single P–H bonds; the remaining two form one lone pair on phosphorus.
Is the electron-domain geometry of PH₃ tetrahedral?
Yes. The basic VSEPR model counts three P–H bonds and one lone pair as four electron groups around phosphorus. These groups have a tetrahedral electron-domain arrangement. The positions of the atoms alone give PH₃ its different, trigonal-pyramidal molecular geometry.
Why isn’t PH₃’s bond angle 109.5°?
The 109.5° angle is the ideal angle for a tetrahedral arrangement. It is not a rule that sets the measured bond angle in every molecule with four electron groups. PH₃’s observed H–P–H angle is about 93.5°.
Are P–H bonds polar?
P–H bonds are nearly nonpolar because phosphorus and hydrogen have almost equal Pauling electronegativity values: about 2.19 and 2.20. Their small difference does not make PH₃ nonpolar by itself. The molecule’s shape and overall dipole must also be considered.
What is the dipole moment of PH₃?
PH₃ has a small molecular dipole moment of about 0.58 D. A debye is a unit for measuring dipole moments. The nonzero value shows that the molecule’s total charge distribution is not perfectly balanced, even though its P–H bonds are nearly nonpolar.
Do the bond dipoles in PH₃ cancel?
No. The P–H bond dipoles are small, but their directions in the trigonal-pyramidal structure do not cancel fully. Their combined effect is a small molecular dipole. This is why PH₃ is described as weakly polar rather than nonpolar.
(This article was written by one of our staff writers, Alan Westbrook. Visit our Meet the Team page.)
