The Lewis structure for CH3Br has the carbon in the center with four electrons around it and the three hydrogens and bromine each sharing a single electron with the carbon. The bromine has an additional three pairs of electrons surrounding it.
Lewis structures show how atoms bond to each other to form a complete molecule, and they are useful for demonstrating bonding in basic molecules, such as bromomethane. In some cases, lines are used to indicate bonds, but dots are more common when dealing with basic compounds. In addition to bonding electrons, these diagrams also show nonbonding electrons, such as in the case of bromine in bromomethane. Bromine, in this instance, has three pairs of electrons that are not actively involved in the bonding between it and carbon.
In general, Lewis structures demonstrate that most atoms, with the exception of hydrogen, form molecules to get a total of eight electrons in their outer shells. However, these structures can also be used to hint at the overall shape of a molecule. In the case of bromomethane, the four atoms around the carbon must be arranged fairly evenly in a 3D space, so they form a tetrahedral structure. However, because bromine forms such a long (and therefore weaker) bond, the hydrogens have a little more space than in methane, resulting in a slightly distorted tetrahedral.