Chemical equations need to be balanced not only in terms of the number of atoms, but also by the charge. In other words, just as teh same number of atoms should appear before and after the arrow of an equation, so should the same charge. Take this into account to balance the following chemical equation:
Sn^+2 + Ag -> Sn +Ag ^+
Recognize that there is a total charge of +2 with the reactants, but a total charge of +1 with the products, which is not balanced. To balance the charges, we need two silver ions, Ag +. But to account for TWO silver ions, we also need two silver atoms, Ag.