Answer
The student can't change the subscript on the chlorine atom from a 2 to a 3 because chlorine in its natural state is a diatomic molecule and has two atoms of chlorine bonded to it. This is the elemental form of chlorine and cannot be manipulated.
Work Step by Step
The correct balanced chemical equation for the reaction is:
$2 Al_{(s)} + 3Cl_{2},_{(g)} --> 2AlCl_{3},_{(s)}$
