The molecular geometry of PBr3 (phosphorus tribromide) is trigonal pyramidal. The molecule has three bromine atoms bonded to a central phosphorus atom, with one lone pair of electrons sitting on top, pushing the three bromines downward into a pyramid shape. The measured bond angle between bromine atoms is about 101°, noticeably smaller than the 109.5° you’d expect from a perfect tetrahedron.
Lewis Structure and Electron Count
To figure out the geometry, start with the Lewis structure. Phosphorus contributes 5 valence electrons, and each of the three bromine atoms contributes 7, giving a total of 26 valence electrons. Three of those electron pairs form single bonds between phosphorus and each bromine. One pair remains on phosphorus as a lone pair. The remaining electrons sit on the bromine atoms as lone pairs, completing their octets.
The key detail is what’s happening around the central phosphorus atom: it has four electron groups total (three bonding pairs and one lone pair). In VSEPR notation, this is classified as AX₃E, where A is the central atom, X represents bonded atoms, and E represents lone pairs.
Why It’s Pyramidal, Not Flat
Four electron groups around a central atom arrange themselves in a tetrahedral pattern to minimize repulsion. That’s the electron geometry of PBr3: tetrahedral. But molecular geometry only describes where the atoms are, not where lone pairs sit. Since one of those four positions is occupied by an invisible lone pair rather than a bromine atom, the visible shape is a three-sided pyramid with phosphorus at the top and the three bromines forming the base.
This is the same geometry as ammonia (NH₃), which also has three bonds and one lone pair. The lone pair takes up more space than a bonding pair because it’s held closer to the central atom and spreads out more. That extra repulsion squeezes the three Br-P-Br bond angles down from the ideal tetrahedral angle of 109.5° to just 101°, according to NIST experimental measurements. All three bond angles in PBr3 are identical at 101°, reflecting the molecule’s symmetry.
Bond Length
Each phosphorus-bromine bond in PBr3 measures 2.220 angstroms (222.0 picometers), based on experimental data from NIST. That’s slightly longer than the P-Br bond in phosphorus monobromide (2.171 angstroms), which makes sense: in PBr3, phosphorus is sharing its electrons with three bromine atoms instead of one, slightly weakening each individual bond.
Polarity
The trigonal pyramidal shape makes PBr3 a polar molecule. Each P-Br bond is polar on its own because bromine is more electronegative than phosphorus, pulling electron density toward itself. If the molecule were flat and symmetrical (like the trigonal planar shape of BF₃), those individual bond dipoles would cancel out. But in a pyramid, they don’t. The three bond dipoles all point partially in the same direction, away from the lone pair, creating a net dipole moment. The lone pair reinforces this by adding its own region of electron density on one side of the molecule.
Physical Properties
At room temperature, PBr3 is a colorless to pale yellow fuming liquid with a sharp, penetrating odor. It boils at about 173°C (343°F) and freezes at roughly -41.5°C (-40°F). It’s corrosive to both metals and tissue, and it reacts vigorously with water.
Why PBr3 Matters in Chemistry
Phosphorus tribromide is one of the standard reagents in organic chemistry for converting alcohols into alkyl bromides. It replaces the hydroxyl group (-OH) on an alcohol with a bromine atom, which is a much better leaving group for further reactions. This conversion works well for primary and secondary alcohols and proceeds through a mechanism that inverts the stereochemistry at the carbon, meaning the bromine ends up on the opposite side from where the -OH was. That predictable inversion is one reason PBr3 is so widely used in synthesis.