In the periodic table the groups are vertical and the horizontal periods. The elements of group 13 such as Aluminum (Al), Boron and others. These elements have 3 valence electrons. Element X needs to have 3 valence electrons since Br accepts an electron and there are 3 bromine.
A controlled experiment is one in which evrerything is held constant except for one verieble, maybe is usually a st of data is taken for a control group.
