To draw the Lewis dot structure for phosphorus tribromide (PBr3), we start by identifying the valence electrons. Phosphorus has 5 valence electrons, and bromine has 7 valence electrons; thus, with three bromine atoms, we have:
5 (from P) + 3 × 7 (from 3 Br) = 26 valence electrons.
In the Lewis structure, the phosphorus atom will be at the center with three bromine atoms surrounding it. Each bromine atom forms a single bond with phosphorus. After forming these three P-Br bonds, we have used 6 electrons (3 bonds × 2 electrons per bond), leaving us with 20 electrons. These remaining electrons are placed as lone pairs on the bromine atoms, giving each bromine three lone pairs of electrons.
The Lewis structure can be represented as follows:
Br
|
Br-P-Br
|
Br
Next, we determine the electron geometry. PBr3 has four regions of electron density (three P-Br bonds and one lone pair on phosphorus), which corresponds to a tetrahedral electron geometry.
However, because of the lone pair, the molecular shape is described as trigonal pyramidal. This shape is similar to that of ammonia (NH3), where the lone pair occupies more space and pushes the bonded bromine atoms closer together.
As for polarity, the molecule is polar. Phosphorus is less electronegative than bromine, resulting in polar P-Br bonds. The trigonal pyramidal shape means that the dipole moments do not cancel out, leading to an overall molecular dipole. Therefore, PBr3 is a polar molecule.