Chemistry: The Central Science (13th Edition)

$HgBr_2$
Mercury can have a variety of different charges. However, the textbook specifies that it is Mercury (II). The (II) indicates that this Mercury ion has a charge of 2+. Bromine is located one group to the left of the noble gases, meaning that it has a charge of 1-. In order for the charges of the ions to cancel each other out, there must be two Bromine ions for every one Mercury ion. Because metals come before non-metals in chemical formulas, the chemical formula for Mercury (II) Bromide is $HgBr_2$.