Answer
$Mg(s)+Cl_2(g)\rightarrow MgCl_2(s)$
Work Step by Step
The balanced equation for the reaction is
$Mg(s)+Cl_2(g)\rightarrow MgCl_2(s)$
Mg is in the 2nd column of the periodic table so its ion has a +2 charge. Cl is in the 2nd column from the right, so its ion has a -1 charge. Mg and Cl combine in a 1:2 ratio to balance the charges.