The Lewis structure for boron tribromide is drawn with a B in the center with three lines connecting to three Brs, each of which contains three pairs of dots around it. The B is the symbol for the single boron atom, and the Brs are the symbol for the three bromine atoms in boron tribromide.
Each of the dots represents a valence electron. Each line represents a bonded pair of electrons that allow the different atoms to combine together to form a single molecule. Separated, each bromine atom contains seven valence electrons, and each boron atom contains three valence electrons, leading to a total of 24 valence electrons. This molecule's configuration of electrons is stable, even though it leaves boron with only six electrons in its outer shell.
The Lewis dot structure illustrates these 24 electrons through the dots and lines. Instead of dots, each pair of electrons can also be represented by a line. Conversely, all bonding lines can be represented by dots. The formal charge of an atom can be determined using the Lewis dot structure by taking the total number of valence electrons and subtracting from that the non-bonding valence electrons. Subtracting half the number of bonding electrons from that number gives the formal charge.