Here's why:

* Zn represents the element Zinc.

* Cl represents the element Chlorine.

* The subscript 2 indicates that there are two chlorine atoms for every one zinc atom.

This is due to the charges of the ions:

* Zinc has a +2 charge (Zn²⁺)

* Chlorine has a -1 charge (Cl⁻)

To balance the charges, you need two chloride ions (2 x -1 = -2) to equal the +2 charge of the zinc ion.