In the reaction ( \text{Mg}(s) + \text{Br}_2(l) \rightarrow \text{MgBr}_2(s) ), the coefficient for ( \text{Br}_2 ) should be 1. This is because one magnesium atom reacts with one molecule of bromine to produce one formula unit of magnesium bromide, ( \text{MgBr}_2 ). Thus, the balanced equation is ( 1 \text{Mg}(s) + 1 \text{Br}_2(l) \rightarrow 1 \text{MgBr}_2(s) ).