Final answer:
Using the VSEPR theory, PBr3 has a tetrahedral electronic geometry with four regions of electron density, but the molecular geometry is trigonal pyramidal due to the presence of one lone pair on the central phosphorus atom.
Step-by-step explanation:
The electronic geometry of PBr3 can be determined using the Valence Shell Electron Pair Repulsion (VSEPR) theory, which helps predict the shape of molecules based on the repulsion between electron pairs around a central atom. In PBr3, the central phosphorus atom has three bonded pairs with the bromine atoms and one lone pair of electrons. According to the VSEPR theory, these four regions of electron density around the central phosphorus atom would be arranged in a tetrahedral fashion to minimize repulsions. However, since there is one lone pair, the molecular geometry is trigonal pyramidal, not tetrahedral. The lone pair occupies one vertex of the tetrahedral and pushes the bonding pairs slightly closer together, resulting in slightly smaller bond angles than the ideal 109.5° found in a perfect tetrahedral geometry.